configure.ac: Check for poll.h, and for nfds_t on Darwin

configure.ac: Check for poll.h, and for nfds_t on Darwin

On Linux, assume nfds_t is always available.

On Darwin, fall back to unsigned int when poll() exists but there
is no nfds_t, such as on Mac OS X before 10.4.

On Windows (both MinGW and Cygwin), always use unsigned int instead
of nfds_t, and don't check for poll.h because we use our own poll()
implementation.