Add PHP mail support

[pjeby]

Basic idea: Use sSMTP as a transport, by making the ssmtp package part of the default image, and including the necessary PHP config if SSMTP_MAILHUB is set. There'd be a template in tpl/etc/ssmtp/ssmtp.conf to configure (roughly):

Mailhub={{SSMTP_MAILHUB}}
FromLineOverride={{SSMTP_FROM_LINE_OVERRIDE}}
UseTLS={{SSMTP_USE_TLS}}
UseSTARTTLS={{SSMTP_USE_STARTTLS}}
AuthUser={{SSMTP_USER}}
AuthPass={{SSMTP_PASS}}
AuthMethod={{SSMTP_AUTH_METHOD}}

There might need to be a Hostname setting, too, and defaults need to be decided on. Plus many of the above need if's to only include those lines if the variable has a value. There's also a few other options, like RewriteDomain and root. (A related issue is logging: an rsyslog install would be needed, configured to send stuff to stdout or stderr. That's a whole other feature by itself.)

[pjeby]

Closing as virtually everything I work with these days has its own way of configuring mail/SMTP, independent of PHP, and there's too many variables in ssmtp config to make this worth doing.