server/peer.c: Engage the interval timer only while there are peers.

server/peer.c: Engage the interval timer only while there are peers.

The reason we need the interval timer is (a) to top up the entropy pool,
and (b) to clean up old keysets.  If there are no peers, we do no key
exchange, so we don't need any random bits, and there are no keysets to
clear away.  So we can save wakeups in this case.

For now, there are stub versions of `iv_(add|rm)reason' in `test.c', but
these will vanish once we split `main' out into its own file.  (Spoiler
alert!)