Implement the memset_s(3) function as specified by the C11 ISO/IEC
9899:2011 Appendix K 3.7.4.1.

MFC r316258:
Only activate __EXT1_VISIBLE block when using sys/errno.h in userspace.

Export __cxa_thread_atexit_impl as an alias for __cxa_thread_atexit.

Add __cxa_thread_atexit(3) API implementation.

Add a manpage for it, assign the copyright to the OpenBSD project on it since it
is mostly copy/paste from OpenBSD manpage.
style(9) fixes

Differential Revision:	https://reviews.freebsd.org/D2420
Reviewed by:	kib

(or loading a dso linked to libthr.so into process which was not
linked against threading library).

- Remove libthr interposers of the libc functions, including
  __error(). Instead, functions calls are indirected through the
  interposing table, similar to how pthread stubs in libc are already
  done.  Libc by default points either to syscall trampolines or to
  existing libc implementations.  On libthr load, libthr rewrites the
  pointers to the cancellable implementations already in libthr.  The
  interposition table is separate from pthreads stubs indirection
  table to not pull pthreads stubs into static binaries.

- Postpone the malloc(3) internal mutexes initialization until libthr
  is loaded.  This avoids recursion between calloc(3) and static
  pthread_mutex_t initialization.

- Reinstall signal handlers with wrapper on libthr load.  The
  _rtld_is_dlopened(3) is used to avoid useless calls to sigaction(2)
  when libthr is statically referenced from the main binary.

In the process, fix openat(2), swapcontext(2) and setcontext(2)
interposing.  The libc symbols were exported at different versions
than libthr interposers.  Export both libc and libthr versions from
libc now, with default set to the higher version from libthr.

Remove unused and disconnected swapcontext(3) userspace implementation
from libc/gen.

No objections from:	deischen
Tested by:	pho, antoine (exp-run) (previous versions)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

bsearch_b is the Apple blocks enabled version of bsearch(3).
This was added to libc in Revision 264042 but the commit
missed the declaration required to make use of it.

While here move some other block-related functions to the
BSD_VISIBLE block as these are non-standard.

Phabric:	D638
Reviewed by:	theraven, wollman

While testing this I found a conformance issue in hdestroy()
that will be fixed in a subsequent commit.

Obtained from:	NetBSD (hcreate.c, CVS Rev. 1.7)

intended to build with any C compiler.

Reviewed by:	pfg
MFC after:	3 weeks

prior to 3.0.0 release) as contrib/jemalloc, and integrate it into libc.
The code being imported by this commit diverged from
lib/libc/stdlib/malloc.c in March 2010, which means that a portion of
the jemalloc 1.0.0 ChangeLog entries are relevant, as are the entries
for all subsequent releases.