Merge the sigaction() and sigprocmask() overloads/wrappers from libpthread
into libc, and move pthread_sigmask() as well (just a trivial wrapper).
This provides consistent handling of SIGTHR between single- and multi-threaded
programs and is a step in the merge of all the libpthread overloads, providing
some ASM and Makefile bits that the other wrappers will need.

ok deraadt@ millert@