lib: additional thread.c MT-safety work

lib: additional thread.c MT-safety work

Fixes a few insufficient critical sections. Adds back locking for
thread_cancel(), since while thread_cancel() is only safe to call from
the pthread which owns the thread master due to races involving
thread_fetch() modifying thread master's ready queue, we still need
mutual exclusion here for all of the other public thread.c functions to
maintain their MT-safety.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>