agetty: don't ignore netlink on select()

agetty: don't ignore netlink on select()

agetty uses NETLINK_ROUTE to be notified about network interface
changes. Unfortunately, the code that monitor the netlink FD does not
increment number of the monitored file descriptors when call
select(2), so the netlink notifications are invisible for agetty.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1278906
Signed-off-by: Karel Zak <kzak@redhat.com>