Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r256661 r257222 r257235 r257248 r257298.

MFC r256661:
Fix the libproc build when DEBUG is defined.

MFC r257222:
Clean up the debug printing in libproc a bit. In particular:

* Don't print any error messages to stderr unless DEBUG is defined.
* Add a DPRINTFX macro for use when errno isn't set.
* Print the error string from libelf when appropriate.

MFC r257235:
Remove an incorrect debug printf.

MFC r257248:
Fix the build with gcc.

MFC r257298:
Revert r257248 and fix the problem in a way that doesn't violate style(9).

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Ignore EINTR when calling waitpid.

Several fixes for libproc:
o return the correct status in proc_wstatus()
o proc_read takes a void *
o correctly allocate the objs structure array

Sponsored by: The FreeBSD Foundation

New version of libproc. Changes are:
* breakpoint setup support
* register query
* symbol to address mapping and vice-versa
* more misc utility functions based on their Solaris counterpart

Also, I've written some test cases.

Sponsored by: The FreeBSD Foundation

Add a process library with some stubs that the DTrace client needs.
These will be fleshed out as part of the DTrace userland tracing
development.

For now, the kernel tracing part of DTrace requires minimal functionality
for this library.

The API for this library is deliberately different from the libproc in
OpenSolaris due to licensing restrictions.