Extend imsg and ibuf API with useful getter methods

For ibufs:
- various getters for ibufs (ibuf_get* and ibuf_skip)
- additional ibuf set/add functions that don't alter byte order
- ibuf_truncate and ibuf_rewind
- ibuf_from_buffer and ibuf_from_ibuf to populate a reader ibuf
- a getter for the msgbuf queuelen

For imsg:
- various getters for imsg (especially imsg_get_data() which can be used
in most cases as a simple one call api with all error checks).
All the imsg.hdr fields can also be accessed by getters.
- The imsg data is now actually an ibuf but the old imsg.data pointer is
kept for now to not break every imsg application.
- Introduce imsg_forward to simply forward a message from one channel to
an other (used in the control socket code).

Since this requires a major bump take the oportunity to also cleanup some
function signatures to use size_t for length fields. Also internal data
structures are removed from the public header.

With and OK tb@

Improve the ibuf API by adding these functions:
Functions extending ibuf_add to work with more specific data types
ibuf_add_buf, ibuf_add_zero, ibuf_add_n8, ibuf_add_n16, ibuf_add_n32,
ibuf_add_n64
Functions replacing ibuf_seek where data at a specific offset is modified
ibuf_set, ibuf_set_n8, ibuf_set_n16, ibuf_set_n32, ibuf_set_n64
Functions to check, get and set the filedescriptor stored on the ibuf
ibuf_fd_avail, ibuf_fd_get, ibuf_fd_set
and ibuf_data() to access the data buffer, to be used together with ibuf_size()

On top of this add an optimized imsg_compose_ibuf() where an ibuf is wrapped
into an imsg in an efficent way.

Finally remove msgbuf_drain since it is not used by anything outside of
the ibuf code. Because of this removal bump the major of libutil.

Remove ibuf_data() in iked since the same function is now provided by libutil.
OK tb@

Make imsg.h a bit more self-contained by prototyping struct iovec.
OK martijn@ mvs@ deraadt@

Change imsg header definitions to use standard types.

ok deraadt@ claudio@

Use C99 types (uint32_t) instead of BSD (u_int32_t) - the former are
more portable. Add stdint.h to the headers in imsg_init(3).

No objections from millert@.

constify data parameter in imsg_add() and imsg_compose()

ok deraadt@

Protect against multiple inclusion, from Christiano F. Haesbaert.

ok deraadt@

Move imsg into libutil and add a man page.

Minor bump for libutil.

Previous versions of this diff and man page looked at by various people.

"you should just commit" deraadt

Improve the ibuf API by adding these functions:
Functions extending ibuf_add to work with more specific data types
ibuf_add_buf, ibuf_add_zero, ibuf_add_n8, ibuf_add_n16, ibuf_add_n32,
ibuf_add_n64
Functions replacing ibuf_seek where data at a specific offset is modified
ibuf_set, ibuf_set_n8, ibuf_set_n16, ibuf_set_n32, ibuf_set_n64
Functions to check, get and set the filedescriptor stored on the ibuf
ibuf_fd_avail, ibuf_fd_get, ibuf_fd_set
and ibuf_data() to access the data buffer, to be used together with ibuf_size()

On top of this add an optimized imsg_compose_ibuf() where an ibuf is wrapped
into an imsg in an efficent way.

Finally remove msgbuf_drain since it is not used by anything outside of
the ibuf code. Because of this removal bump the major of libutil.

Remove ibuf_data() in iked since the same function is now provided by libutil.
OK tb@

Make imsg.h a bit more self-contained by prototyping struct iovec.
OK martijn@ mvs@ deraadt@

Change imsg header definitions to use standard types.

ok deraadt@ claudio@

Use C99 types (uint32_t) instead of BSD (u_int32_t) - the former are
more portable. Add stdint.h to the headers in imsg_init(3).

No objections from millert@.

constify data parameter in imsg_add() and imsg_compose()

ok deraadt@

Protect against multiple inclusion, from Christiano F. Haesbaert.

ok deraadt@

Move imsg into libutil and add a man page.

Minor bump for libutil.

Previous versions of this diff and man page looked at by various people.

"you should just commit" deraadt

Make imsg.h a bit more self-contained by prototyping struct iovec.
OK martijn@ mvs@ deraadt@

Change imsg header definitions to use standard types.

ok deraadt@ claudio@

Use C99 types (uint32_t) instead of BSD (u_int32_t) - the former are
more portable. Add stdint.h to the headers in imsg_init(3).

No objections from millert@.

constify data parameter in imsg_add() and imsg_compose()

ok deraadt@

Protect against multiple inclusion, from Christiano F. Haesbaert.

ok deraadt@

Move imsg into libutil and add a man page.

Minor bump for libutil.

Previous versions of this diff and man page looked at by various people.

"you should just commit" deraadt

Change imsg header definitions to use standard types.

ok deraadt@ claudio@

Use C99 types (uint32_t) instead of BSD (u_int32_t) - the former are
more portable. Add stdint.h to the headers in imsg_init(3).

No objections from millert@.

constify data parameter in imsg_add() and imsg_compose()

ok deraadt@

Protect against multiple inclusion, from Christiano F. Haesbaert.

ok deraadt@

Move imsg into libutil and add a man page.

Minor bump for libutil.

Previous versions of this diff and man page looked at by various people.

"you should just commit" deraadt

Use C99 types (uint32_t) instead of BSD (u_int32_t) - the former are
more portable. Add stdint.h to the headers in imsg_init(3).

No objections from millert@.

constify data parameter in imsg_add() and imsg_compose()

ok deraadt@

Protect against multiple inclusion, from Christiano F. Haesbaert.

ok deraadt@

Move imsg into libutil and add a man page.

Minor bump for libutil.

Previous versions of this diff and man page looked at by various people.

"you should just commit" deraadt