Do not allow receipt of mail for built-in sysop aliases (i.e. "sysop" and

"postmaster") unless the ALLOW_SYSOP_ALIASES option is enabled
(disabled by default) or the alias is specifically listed in the ctrl/alias.cfg
file.
Also, if the alias.cfg file correlates a user *number* with an alias, support
receipt of mail to that alias even if the RX_BY_NUMBER option is not enabled.
This allows us to use '1' instead of 'sysop' in the default alias.cfg file.