This patch allows for configuring the specific user, group, and whether
to set the setuid and setgid bits.

See also:
  https://lists.gnu.org/archive/html/guix-devel/2020-11/msg00369.html

But I thought I'd open this here so we could track changes since this is
technically independent of the postfix stuff.  Anyway, patch attached.
One change since the last email above is that I added support for
string-based username/groups.

This also needs documentation, I suppose, so that should be done.
But it would be good to know if this patch looks like it's on the "right
path" or not.