add C11 thread functions operating on tss_t and once_flag

These all have POSIX equivalents, but aside from tss_get, they all
have minor changes to the signature or return value and thus need to
exist as separate functions.

use weak symbols for the POSIX functions that will be used by C threads

The intent of this is to avoid name space pollution of the C threads
implementation.

This has two sides to it. First we have to provide symbols that wouldn't
pollute the name space for the C threads implementation. Second we have
to clean up some internal uses of POSIX functions such that they don't
implicitly drag in such symbols.

optimize POSIX TSD for fast pthread_getspecific

initial check-in, version 0.5.0