Add a pselect emulation.

* src/npth.c [!HAVE_PSELECT]: Include signal.h
(npth_pselect) [!HAVE_PSELECT]: Implement a crude pselect emulation
* configure.ac (AC_CHECK_FUNCS): Test for pselect.

This is a better than nothing emulation. OpenBSD applications who
care about race conditions will probably use their own signal detection
via pipe. This can be implemented on top of this emulation.