Implement SMTP AUTH LOGIN and AUTH PLAIN extensions for authentication

Implement SMTP AUTH LOGIN and AUTH PLAIN extensions for authentication
when sending mail by SMTP.  New Edwin variable MAIL-AUTHENTICATION
controls whether to authenticate; set it to #T to enable authentication
by default.  New Edwin variable SMTP-USER-NAME can be set to specify a
user name.

Still to do:  CRAM-MD5 (shouldn't be hard, but I don't need it now),
and automatic detection of necessity of authentication.  Is there an
SMTP reply code to tell us that we must authenticate?