Move some pthread functions to signal.h.

POSIX specifies that pthread_kill(3) and pthread_sigmask(3) are
supposed to live in signal.h rather than pthread.h.

Since signal.h now needs pthread_t and pthread_attr_t, I've moved
those defintions into include/machine/pthread_types.h to keep the
namespace clean. I also sorted some includes. The combination of these
two things seems to have exploded into a cascade of missing includes,
so this patch also cleans up all those.

Change-Id: Icfa92a39432fe83f542a797e5a113289d7e4ad0c
Merge "Move some pthread functions to signal.h."