This has been sitting in my local tree long enough. Remove the use

This has been sitting in my local tree long enough.  Remove the use
of an alternate signal stack for handling signals.  Let the kernel
send signals on the stack of the current thread and teach the threads
signal handler how to deliver signals to the current thread if it
needs to.  Also, always store a threads context as a jmp_buf.  Eventually
this will change to be a ucontext_t or mcontext_t.

Other small nits.  Use struct pthread * instead of pthread_t in internal
library routines.  The threads code wants struct pthread *, and pthread_t
doesn't necessarily have to be the same.

Reviewed by:	jasone