This is a patch that fixes "<executable name>: this program must be setgid smtpq".
* gnu/services/mail.scm (<opensmtpd-configuration>)[setgid-commands?]: New field.
(opensmtpd-set-gids): New procedure.
(opensmtpd-service-type)[extensions]: Add SETUID-PROGRAM-SERVICE-TYPE extension.
* doc/guix.texi (Mail Services): Document it.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>