lib: Display to end user the MAX_FDS allowed

lib: Display to end user the MAX_FDS allowed

Upon startup FRR reads in the MAX_FDS variable from
it's control files via the getrlimit call.  We then
setup code to limit the poll data structure size to
that value.  The OS also limits our FD's to that value
because that is what is set.  Provide a methodology
that a interested end user can figure this data out.

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