Creating a mailbox room no longer automatically sets

Creating a mailbox room no longer automatically sets
the 'subject required' bit.  Instead, the server recommends a subject at ENT0 time
if the 'subject required' bit is set *or* there is at least one Internet email
recipient.  This allows local citadel users to send mail to each other without
subjects, which is customary.
* Also made the ENT0 result for this function numeric instead of a string.