MFC r337992, r338125:
POSIX compliance improvements in the pthread(3) functions.

This basically adds makes use of the C99 restrict keyword, and also
adds some 'const's to four threading functions: pthread_mutexattr_gettype(),
pthread_mutexattr_getprioceiling(), pthread_mutexattr_getprotocol(), and
pthread_mutex_getprioceiling. The changes are in accordance to POSIX/SUSv4-2018.

Hinted by: DragonFlyBSD

Relnotes: yes

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Remove mentions that PTHREAD_PROCESS_SHARED is not supported from man pages.

Also add support for process-shared synchronization objects to the HISTORY
sections where they already exist.

sort SEE ALSO

Hide the implementation details about multiple threading libraries
from the synopses of pthread*(3) manpages.

Reviewed by: deischen, davidxu

Remove mentions of libc_r.

Sort sections.

Mechanically kill hard sentence breaks and double whitespaces.

Document interfaces that are supported by libthr.

Have a single set of POSIX threads man pages. The LIBRARY section
of each manpage lists libraries that have corresponding interfaces
implemented.

Prodded by: threads
Reviewed by: deischen

mdoc(7) police: Use the new .In macro for #include statements.

mdoc(7) police: sort SEE ALSO xrefs (sort -b -f +2 -3 +1 -2).

Introduce .Lb macro to libc_r manpages.

$Id$ -> $FreeBSD$

Implement pthread read/write locks as defined by Version 2 of the Single
UNIX Specification.

As with our standard mutexes, process shared locks are not supported at
this time.