Number of subscriptions can be checked

The number of subscribed clients can be set in
the director ressource, e.g.

Subscriptions = 2

When the number of clients is bigger than the
configured limit, each job gets the following
message:

JobId 7: Warning: Subscriptions exceeded: (used/total) (3/2)

Also, the status of the subscriptions can be checked via
* status subscriptions
Warning! No available subscriptions: -1 (3/2) (used/total)

Nothing else than the warning is issued, no enforcement on
backup, restore or any other operation will happen.

Fixes #152: report if number of subscribed clients is exceeded.

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>