pthreads - Fix rtld-elf and libthread_xu

* Fixes chrome, thunderbird, and multiple other issues with recent
libpthreads work.

Testing-by: mneumann, dillon

libthread_xu - Fix rtld and refactor locks

* Add a separate atfork facility for internal pthread atfork entities
(sem and rtld) which must execute after all user atfork entities
pre-fork and bef

libthread_xu - Fix rtld and refactor locks

* Add a separate atfork facility for internal pthread atfork entities
(sem and rtld) which must execute after all user atfork entities
pre-fork and before all user atfork entities post-fork.

* Install an atfork handler for rtld-elf (also requires rtld-elf to
be updated). The handler will ensure that RTLD locks are in a sane
state prior to fork (by acquiring them), and will then release the
locks post-fork. This is the primary fix for lang/rust and cargo.

Also do not issue _thr_rtld_fini() when threading drops to 0.
Once threading has been set, rtld's pthread locks remain installed.

* Refactor thr_cond.c. Refactor condition variables to perform
according to the spec. Use a TAILQ to make pthread_cond_signal()
work exactly as described in the manual (that is, waking up only
one waiter at a time).

* Refactor thr_mutex.c. Primary instrument for debugging and
clean up. Also deal with improper EINTR handling.

* Refactor thr_fork.c. Implement the new atfork facility for
internal atfork handlers.

* Refactor thr_rwlock.c. Add debugging, cleanup.

* thr_sem.c now uses the internal atfork handler to ensure proper
ordering.

* thr_sig.c implements debugging features.

* Refactor thr_umtx.c... the low level mutex code. Store the id
for additonal verification and use an atomic lock to clear the
lock instead of an assignment. Properly ignore EINTR.

* Cleanup init_private() a bit.

* Add PTHREADS_DEBUGGING=TRUE and PTHREADS_DEBUGGING2=TRUE make
flags. The first writes out a garbage file in /tmp for all
locking operations as they occur. The second is used for
point debugging and writes out a file when signal 63 is sent
to the program.

* Add cpu_ccfence() in various places that might need it (a hack
for the moment, userland cannot currently #include
"machine/cpufunc.h").

* Should fix lang/rust and 'cargo'

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libc/libpthread: Inject threadsafe locking callbacks for rtld.

* Make rtld thread-safe. Add lib/libthread_xu/thread/thr_rtld.c
to the build and use it to inject pthread based locking callbacks
f

libc/libpthread: Inject threadsafe locking callbacks for rtld.

* Make rtld thread-safe. Add lib/libthread_xu/thread/thr_rtld.c
to the build and use it to inject pthread based locking callbacks
for rtld.

* In rtld_lock.c, do the thread_mask_clear(lock->mask); after releasing
the actual lock, to avoid deadlocks when rtld is called recursively
from the pthread locking callbacks.
XXX An alternative solution for this issue would be, to flag the
libpthread dynamic library as bind_now to avoid the recursive rtld
calls.

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pthread: General pre-cleanup (style, typos etc)

No functional change.

pthread: Sync copyright changes with FreeBSD.

Taken-from: FreeBSD

Cleanup the TLS implementation:
- Define the TCB layout in machine/tls.h, define wether tcb_self exists or
not.
- Define the layout of the DTV, not used yet.
- Define the TLS variant
- Define commo

Cleanup the TLS implementation:
- Define the TCB layout in machine/tls.h, define wether tcb_self exists or
not.
- Define the layout of the DTV, not used yet.
- Define the TLS variant
- Define common access functions (tls_get_tcb, tls_get_curthread, tls_set_tcb)
and the RTLD functions.

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Import initial version of 1:1 pthread library.