gnu: Add dovecot service

gnu: Add dovecot service

* gnu/services/mail.scm: New file.
(&dovecot-configuration-error, dovecot-configuration-error?)
(dovecot-service, dovecot-configuration, dict-configuration)
(passdb-configuration, userdb-configuration)
(unix-listener-configuration, fifo-listener-configuration)
(inet-listener-configuration, service-configuration)
(protocol-configuration, plugin-configuration, mailbox-configuration)
(namespace-configuration, opaque-dovecot-configuration): New public
variables.

* gnu-system.am (GNU_SYSTEM_MODULES): Add (gnu services mail).

* doc/guix.texi (Mail Services): New node.