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

Revert r330897:

This was intended to be a non-functional change. It wasn't. The commit
message was thus wrong. In addition it broke arm, and merged crypto
related code.

Revert with prejudice.

This revert skips files touched in r316370 since that commit was since
MFCed. This revert also skips files that require $FreeBSD$ property
changes.

Thank you to those who helped me get out of this mess including but not
limited to gonzo, kevans, rgrimes.

Requested by: gjb (re)

Partial merge of the SPDX changes

These changes are incomplete but are making it difficult
to determine what other changes can/should be merged.

No objections from: pfg

MFC r318539 r318580 r318581 r318584 r318749 r318952 r318953 r318955

libthr: fix warnings at WARNS=6

Fix warnings about the following when WARNS=6 (which I will commit soon):

- casting away const
- no previous 'extern' declaration for non-static variable
- others as explained by #pragmas and comments
- unused parameters

libthr: disable thread-safety warnings

These warnings don't make sense for code that implements
the locking primitives.

libthr: change CHECK_AND_INIT_RWLOCK to an inline function

This was prompted by a compiler warning about 'ret' shadowing
a local variable in the callers of the macro.

libthr: Use CLI flags instead of pragmas to disable warnings

People tweaking the build system or compilers tend to look into
the Makefile and not into the source. Having some warning controls
in the Makefile and some in the source code is surprising.

Pragmas have the advantage that they leave the warnings enabled
for more code, but that advantage isn't very relevant in these cases.

libthr: fix warnings from GCC when WARNS=6

Fix warnings about:
- redundant declarations
- a local variable shadowing a global function (dlinfo)
- an old-style function definition (with an empty parameter list)
- a variable that is possibly used uninitialized

libthr: prevent setcontext() from masking SIGTHR

__thr_setcontext() mistakenly tested for the presence of SIGCANCEL
in its local ucontext_t instead of the parameter. Therefore,
if a thread calls setcontext() with a context whose signal mask
contains SIGTHR (a.k.a. SIGCANCEL), that signal will be blocked,
preventing the thread from being cancelled or suspended.

Sponsored by: Dell EMC

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

Use __FBSDID() for .c files from lib/libthr/thread.

Sponsored by: The FreeBSD Foundation

Use sysctl kern.sched.cpusetsize to retrieve size of kernel cpuset.

Check small set and reject it, this is how kernel did. Always use the
size kernel is using.

- Revert r214409.
- Use long word to figure out sizeof kernel cpuset, hope it works.

Fix typo.

Get cpuset in pthread_attr_get_np() and free it in pthread_attr_destroy().

MFC after: 7 days

Convert thread list lock from mutex to rwlock.

Avoid various shadowed variables. libthr is now almost WARNS=4 clean except
for some const dequalifiers that needs more careful investigation.

Ok'ed by: davidxu

Compile libthr with warnings.

Fix a bug when calculating remnant size.

Check actual size of cpuset kernel is using and define underscore version
of API.

implement pthread_attr_getaffinity_np and pthread_attr_setaffinity_np.

Remove 3rd clause, renumber, ok per email

Use kernel facilities to support real-time scheduling.

- Use same priority range returned by kernel's sched_get_priority_min()
and sched_get_priority_max() syscalls.
- Remove unused fields from structure pthread_attr.

WARNS level 4 cleanup.

Return real detached state.

Import my recent 1:1 threading working. some features improved includes:
1. fast simple type mutex.
2. __thread tls works.
3. asynchronous cancellation works ( using signal ).
4. thread synchronization is fully based on umtx, mainly, condition
variable and other synchronization objects were rewritten by using
umtx directly. those objects can be shared between processes via
shared memory, it has to change ABI which does not happen yet.
5. default stack size is increased to 1M on 32 bits platform, 2M for
64 bits platform.
As the result, some mysql super-smack benchmarks show performance is
improved massivly.

Okayed by: jeff, mtm, rwatson, scottl

Move the weak references to the top of the file to conform
to the format of other similar files in libthr.

- Add libthr but don't hook it up to the regular build yet. This is an
adaptation of libc_r for the thr system call interface. This is beta
quality code.