s3-printing: update parent smbd pcap cache

s3-printing: update parent smbd pcap cache

If a client connects to a samba share and while connected a printer is
added, the client will see the new printer share after a maximum of
'printcap cache time' seconds.

smbd's forked for new client connections inherit printcap information
from the parent (listener) smbd, which does not perform updates on
printcap cache time expiry. Therefore newly connected clients may
initially be presented with stale printer shares.

Add a housekeeping function to the parent smbd to ensure newly connected
clients see up to date printer shares.

The last 2 patches address bug #7836 (A newly added printer isn't visbile to
clients).