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

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

WARNS=3'ify.

Add _pthread_mutex_init_calloc_cb() to libthr and libkse, so that malloc(3)
(part of libc) can use pthreads mutexes without causing infinite recursion
during initialization.

Repo copy libpthreads to libkse.
This introduces the WITHOUT_LIBKSE nob,
and changes WITHOUT_LIBPTHREADS to mean with neither threading libs.
Approved by: re(kensmith)

Don't attempt to initialize the rtld lock if it can't be malloc()'d.

PR: 83452

style(9)

Reviewed by: bde

s/foo()/foo(void)/

Add a blank line after a variable declaration.

Tweak rtld lock to allow recursive on reader lock and detect recursive
on writer lock. This is first cut at rwlock for rtld.

Submitted by: desichen

Temporary disable rwlock based code, replace it with low level KSE locking
code until rtld-elf and libkse can cooperate better, those code can be
restored.

Reviewed by: deischen

Attempt to eliminate PLT relocations from rwlock aquire/release
path, making them suitable for direct use by the dynamic loader.

Register libpthread-specific locking API with rtld on startup.

This still has some rough edges with signals which should be
addresses later.

Approved by: re (scottl)