Note: this change reintroduces r314020 after r314327, r314330, and r314332

MFC r313404:

Improve libnetbsd compatibility with NetBSD

This change is being made to diff reduce/reduce duplication in
contrib/netbsd-tests and to facilitate further porting of software from
NetBSD

Add the following headers:
- sys/event.h:
-- sys/types.h is required for kqueue on FreeBSD, but not NetBSD.
- sys/types.h:
-- NBBY is defined in sys/param.h on FreeBSD, not sys/types.h like on NetBSD.
Pull in sys/param.h to have parity with NetBSD.
- sys/wait.h:
-- Define wrusage as __wrusage for parity with NetBSD typedef.
- glob.h
-- Define __gl_stat_t as "struct stat" for parity with NetBSD typedef.
- pthread.h:
-- Pull in pthread_np.h for _np functions defined separately on FreeBSD.

Improve compatibility with NetBSD in the following headers:

- sha1.h:
-- define SHA1_CTX as SHA_CTX
-- define SHA1Final as SHA1_Final
- sha2.h:
-- #include sha384 to pick up all of the SHA 384 bit macros and definitions.
- util.h:
-- Add sys/types.h to util.h to pollute the header for types used in
flags_to_string and string_to_flags (u_long) as NetBSD doesn't require them
for the functions.

Revert r314020

The test build I ran unfortunately didn't catch the fact that the sha384.h
compat header is missing on ^/stable/10 due to some code not being MFCed

MFC r313404:

Improve libnetbsd compatibility with NetBSD

This change is being made to diff reduce/reduce duplication in
contrib/netbsd-tests and to facilitate further porting of software from
NetBSD

Add the following headers:
- sys/event.h:
-- sys/types.h is required for kqueue on FreeBSD, but not NetBSD.
- sys/types.h:
-- NBBY is defined in sys/param.h on FreeBSD, not sys/types.h like on NetBSD.
Pull in sys/param.h to have parity with NetBSD.
- sys/wait.h:
-- Define wrusage as __wrusage for parity with NetBSD typedef.
- glob.h
-- Define __gl_stat_t as "struct stat" for parity with NetBSD typedef.
- pthread.h:
-- Pull in pthread_np.h for _np functions defined separately on FreeBSD.

Improve compatibility with NetBSD in the following headers:

- sha1.h:
-- define SHA1_CTX as SHA_CTX
-- define SHA1Final as SHA1_Final
- sha2.h:
-- #include sha384 to pick up all of the SHA 384 bit macros and definitions.
- util.h:
-- Add sys/types.h to util.h to pollute the header for types used in
flags_to_string and string_to_flags (u_long) as NetBSD doesn't require them
for the functions.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Add libnetbsd, a thin compatibility layer intended to allow a limited
set of NetBSD software to compile as part of the FreeBSD build with
little or no modifiction. It is built as a static library and not
installed for general use. Likewise, its header files are not
installed.

Sponsored by: DARPA, AFRL