Merge from appliance-head: when creating a default security descriptor

Merge from appliance-head: when creating a default security descriptor
for a printer, save it in ntprinters.tdb instead of recreating it
every time it is required.  This can save at least one winbind lookup
per secdesc creation.  Opening a port monitor and viewing the security
tab in the properties dialog required the security descriptor to be
returned 25 times!
(This used to be commit f85c976acc08a16bce6cb4053708485ccdea0d77)