server: Warns user to run 'quilt stop' when Daemon gets SIGINT
server: Warns user to run 'quilt stop' when Daemon gets SIGINT

Before this commit, if user tries to kill the daemon before running
'kelda stop', the machines would still be active although daemon is
shut down. After this commit, daemon will warn user the number of
active machines after it shuts down, and tell user how to stop them.

LuiseV/quilt,LuiseV/quilt,NetSys/quilt,kelda/kelda,kelda/kelda,LuiseV/quilt,NetSys/quilt,quilt/quilt,NetSys/quilt,NetSys/quilt,NetSys/quilt,quilt/quilt,LuiseV/quilt,LuiseV/quilt,kelda/kelda,quilt/quilt,kelda/kelda,quilt/quilt