If both mail_location and default_mail_env are set in the config file, give an error.
If both mail_location and default_mail_env are set in the config file, give
an error.

--HG--
branch : HEAD

jkerihuel/dovecot,dscho/dovecot,dscho/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,dscho/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,dscho/dovecot,dscho/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch