Added bootstrap/configure option to force pselect

All modern Linux kernels have ppoll() but sometimes
you might want to compile on something ancient.

This patch adds the possibility to force the use
of pselect() instead by passing --force-pselect
to bootstrap/configure.

The use of ppoll() is still default for Linux
and OpenBSD
Merge pull request #583 from tru/add_force_pselect

Added bootstrap/configure option to force pselect