Basic su_port.c implementation using pthreads and poll() is now divided into
three modules: su_base_port.c contains base implementation with su_base_*()
functions, su_pthread_port.c contains pthread-specific implementation and
su_poll_port() contains poll()/epoll()-specific parts. The decomposed
su_port allows implementations on different platforms and usages to share
code as far as possible.
This patch also introduces configure option --disable-poll-port which should
be used on systems with emulated poll, like older *BSD derivatives such as
OS X. Note however that su_select_poll.c is not completed yet.