lib: Add ability to use poll() instead of select

lib: Add ability to use poll() instead of select

This patch originated w/ Hannes Hofer <hhofer@barracuda.com>.
I've taken the patch fixed some bugs and reworked the code
to allow both poll and select to be choosen at compile time.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>