Settings in maia_users don't inherit from domain and system defaults

[rjl]

As reported by Steven Sorrells, the maia_users.quarantine_digest_interval setting is not inherited properly from the domain-default and system-default accounts, as it should be. This problem applies more generally to all settings in the maia_users table, since only settings in the policy table follow the system->domain->user inheritance cascade at the moment.

Any column in the maia_users table that has a DEFAULT clause in its SQL column definition ought to have its value inherited from the domain-default and system-default accounts as expected, with perhaps an exception for the spamtrap column (which should always be set to 'N' by default for safety reasons).

[dmorton]

added in [1098]