Remove _THREAD_SAFE depenendancies. Create weakly associated stubs for

Remove _THREAD_SAFE depenendancies.  Create weakly associated stubs for
posix threading calls used by libc and weak associations for procedures that
a threading library would otherwise have to override.  This allows a program
to be linked with libc, or linked with libc + a threading library.  libc_r
will be deprecated over time.

Traditionally libc_r had to be linked against a threading library rather then
libc.  Linux, FreeBSD, and other UNIXes have moved either t oa thread-safe
libc or a thread-capable libc.  The threading capability is accomplished by
making the appropriate thread locking calls and providing weakly referenced
dummy stub functions for the case where a threading library is not linked in.
Certain third party software, namely XFree/XOrg, also assume threading
stubs in libc.

This is precursor work to a port of David Xu's 1:1 threading library and is
also intended as a basis to allow DragonFly to support multiple threading
libraries.

Ported-by: David Rhodus <sdrhodus@gmail.com>
Primarily-ported-from: FreeBSD
Testing and bug fixes by David Xu and Matt Dillon