lib: allow pthreads to poke poll()

lib: allow pthreads to poke poll()

When scheduling a task onto a thread master owned by another pthread, we
need to lock the thread master's mutex. However, if the pthread which
owns that thread master is in poll(), we could be stuck waiting for a
very long time. To solve this, we copy all data poll() needs and unlock
during poll(). To break the target pthread out of poll(), thread_master
has gained a pipe whose reading end is passed into poll(). After an event
that requires immediate action by the target pthread, a byte is written
into the pipe in order to wake it up.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>
[DL: split off from select() removal]