Tweak status scheduler output to cope with locales.

The %a and %b are locale specific strings which vary in lenght on each
locale. So we now keep track of the longest date string so we can align
the header right. For this to work we print everything into a temporary
buffer and keep track of the length and dump the whole output in one go
with the correct header spacing.
Status scheduler support also view in the past

Now we also can give negative days to the
"status scheduler" command thus see what
schedules were triggered in the past
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>