The user can now send SIGUSR1 to get a snapshot of the current status,
and SIGUSR2 to force reauthentication of all ports (without getting
through stop/start).

Please note that during reauth we do not configure ports to the
auth_vlan network: they would just continue to live on their current
VLAN while the radius server is queried. For those ports where nothing
changed, no service interruption happens.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>