* mts/smtp/smtp.c: added SASL support if mts configuration

* mts/smtp/smtp.c: added SASL support if mts configuration
  setting is set to "sendmail".  This is useful if sendmail
  conf option is to a custom script that creates a proxy
  connection to an smtp server.

* sbr/mts.c: added support for MHMTSCONF and MHMTSUSERCONF
  envvars.  The former specifies an alternative system
  mts.conf to use.  The later specifies a user-specific
  mts.conf to use.  This one will be read after the system
  conf, so the user's conf only needs to set options they
  want to override.  The MHMTSUSERCONF allows a user to set
  personal alternative mail submission methods w/o affecting
  other users on the system.

* uip/whom.c: added SASL-based options so address checking
  can work against a server that requires SASL.