isis: use descriptor polling instead of time

isis: use descriptor polling instead of time

Allow other supported Operating Systems (OS) to use file descriptor
polling, instead of doing timed fd checks. This should improve
performance greatly on modern OSes (e.g. that support polling on
filtered sockets).

The known OS that doesn't support this is FreeBSD < 5.0, but even then
FRR doesn't compile in these versions. OSes using DLPI method (e.g
Solaris) does not support select()/poll()ing fds as well, so it will be
disabled for it.

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>