Mark pthread_equal() as unused inside libc

Revert recent changes to unbreak ports/net/samba

While it is not clear (to me) why that ports ends up with corrupted
shared libs, reverting those changes fixes the issue and should allow us
to close p2k17 more smoothly.

Discussed with a bunch, ok ajacoutot@ guenther@

Change pthread_cleanup_{push,pop} to macros that store the cleanup info
on the stack instead of mallocing the list and move the APIs from libpthread
to libc so that they can be used inside libc.

Note: the standard was explicitly written to permit/support this
"macro with unmatched brace" style and it's what basically everyone
else already does. We xor the info with random cookies with a
random magic to detect/trip-up overwrites.

Major bump to both libc and libpthread due to the API move.

ok mpi@

Move mutex, condvar, and thread-specific data routes, pthread_once, and
pthread_exit from libpthread to libc, along with low-level bits to
support them. Major bump to both libc and libpthread.

Requested by libressl team. Ports testing by naddy@
ok kettenis@

libc.so can't be unloaded, so move the hidden atexit() and pthread_atfork()
stubs for the executable from crtbegin.o into libc, which lets them be
excluded from static links that don't use them.
For this, drop the normal crt{begin,end}S.o from libc.so: the .init and .fini
sections for libc aren't called at the right times anyway, so it's good that
they're unused. libc.so just needs __guard_local and the .note.openbsd.ident
section, so add them to stack_protector.c for now (this will be improved)

"good time" deraadt@

Revert recent changes to unbreak ports/net/samba

While it is not clear (to me) why that ports ends up with corrupted
shared libs, reverting those changes fixes the issue and should allow us
to close p2k17 more smoothly.

Discussed with a bunch, ok ajacoutot@ guenther@

Change pthread_cleanup_{push,pop} to macros that store the cleanup info
on the stack instead of mallocing the list and move the APIs from libpthread
to libc so that they can be used inside libc.

Note: the standard was explicitly written to permit/support this
"macro with unmatched brace" style and it's what basically everyone
else already does. We xor the info with random cookies with a
random magic to detect/trip-up overwrites.

Major bump to both libc and libpthread due to the API move.

ok mpi@

Move mutex, condvar, and thread-specific data routes, pthread_once, and
pthread_exit from libpthread to libc, along with low-level bits to
support them. Major bump to both libc and libpthread.

Requested by libressl team. Ports testing by naddy@
ok kettenis@

libc.so can't be unloaded, so move the hidden atexit() and pthread_atfork()
stubs for the executable from crtbegin.o into libc, which lets them be
excluded from static links that don't use them.
For this, drop the normal crt{begin,end}S.o from libc.so: the .init and .fini
sections for libc aren't called at the right times anyway, so it's good that
they're unused. libc.so just needs __guard_local and the .note.openbsd.ident
section, so add them to stack_protector.c for now (this will be improved)

"good time" deraadt@