Allow to disable a particular print quota entry without deleting it or disabling the entire user account.

[jerome]

To entirely forbid an user to print, one can use pkusers --limitby noprint theUserName, but unfortunately there's currently no way to forbid an user to print but only for a particular printer.

[jerome]

(In [3549]) Removed support for the MaxJobSize? attribute for users group print quota
entries : I couldn't see a real use for this at the moment, and it would
complexify the code. This support might reappear later however. Added full
support for the MaxJobSize? attribute for user print quota entries,
editable with edpykota's new --maxjobsize command line switch. Changed
the internal handling of the MaxJobSize? attribute for printers :
internally 0 used to mean unlimited, it now allows one to forbid
printing onto a particular printer. The database upgrade script (only
for PostgreSQL) takes care of this.
IMPORTANT : the database schema changes. A database upgrade script is
provided for PostgreSQL only. The LDAP schema doesn't change to not
break any existing LDAP directory, so the pykotaMaxJobSize attribute is
still allowed on group print quota entries, but never used.
Seems to work as expected, for a change :-)
Fixes #15.