__pthread_handle_pending_posix_signals() changes

Don't call run_uthread() directly iniside
__pthread_handle_pending_posix_signals().  Actually don't even call this
function 'handle_pending_posix_signals' at all.  Instead call it
'prep_for_pending_posix_signal', and just prep the pthread so it will be
restarted inside its signal handler context next time it is run.
It's semantically identical to what we had before, but it's a better
abstraction since (in the future) we may wish to do something between
this prep and actually restarting the thread.

The previous implementation also forced us to always restore the signal
handler context from pthread->uthread.u_ctx (even if the pthread's
saved context was currently sitting in the vcpd uthread_ctx slot).
This implementation now updates the pthread's current context in place,
allowing it to be restarted the same way whether there were pending
posix signals or not.