Remove $FreeBSD$: two-line nroff pattern

Remove /^\.\\"\n\.\\"\s*\$FreeBSD\$$\n/

More places to clarify the robustness guarantees.

Wording submitted by: Martin Simmons <martin@lispworks.com>
MFC after: 3 days

Merge ^/head r337646 through r338014.

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_getty

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
MFC after: 1 month
Differential Revision: D16722

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Add implementation of robust mutexes, hopefully close enough to the
intention of the POSIX IEEE Std 1003.1TM-2008/Cor 1-2013.

A robust mutex is guaranteed to be cleared by the system upon either
thr

Add implementation of robust mutexes, hopefully close enough to the
intention of the POSIX IEEE Std 1003.1TM-2008/Cor 1-2013.

A robust mutex is guaranteed to be cleared by the system upon either
thread or process owner termination while the mutex is held. The next
mutex locker is then notified about inconsistent mutex state and can
execute (or abandon) corrective actions.

The patch mostly consists of small changes here and there, adding
neccessary checks for the inconsistent and abandoned conditions into
existing paths. Additionally, the thread exit handler was extended to
iterate over the userspace-maintained list of owned robust mutexes,
unlocking and marking as terminated each of them.

The list of owned robust mutexes cannot be maintained atomically
synchronous with the mutex lock state (it is possible in kernel, but
is too expensive). Instead, for the duration of lock or unlock
operation, the current mutex is remembered in a special slot that is
also checked by the kernel at thread termination.

Kernel must be aware about the per-thread location of the heads of
robust mutex lists and the current active mutex slot. When a thread
touches a robust mutex for the first time, a new umtx op syscall is
issued which informs about location of lists heads.

The umtx sleep queues for PP and PI mutexes are split between
non-robust and robust.

Somewhat unrelated changes in the patch:
1. Style.
2. The fix for proper tdfind() call use in umtxq_sleep_pi() for shared
pi mutexes.
3. Removal of the userspace struct pthread_mutex m_owner field.
4. The sysctl kern.ipc.umtx_vnode_persistent is added, which controls
the lifetime of the shared mutex associated with a vnode' page.

Reviewed by: jilles (previous version, supposedly the objection was fixed)
Discussed with: brooks, Martin Simmons <martin@lispworks.com> (some aspects)
Tested by: pho
Sponsored by: The FreeBSD Foundation

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

Reviewed by: deischen, davidxu

Removed the IMPLEMENTATION NOTES section made redundant by not
putting libc_r into the LIBRARY section.

Document interfaces that are supported by libthr.

Spell POSIX threads library as "libpthread", mdoc(7) is smart
enough to render it as "-lkse" these days.

This interface is not provided by libc_r. It is, on the other
hand, is also provided by libthr, but this will be addressed
later.

Aargh, cvs add pthread_mutex_timedlock