services: 'mingetty-service' no longer takes monadic values.

services: 'mingetty-service' no longer takes monadic values.

* gnu/services/base.scm (mingetty-service): Change default value of
  #:motd from a monadic value to a <plain-file>.  Assume MOTD to be a
  file-like object.  Assume LOGIN-PROGRAM is a gexp or #f.
  (%base-services): Use 'plain-file' instead of 'text-file' for motd.
* gnu/system/linux.scm (unix-pam-service): Update docstring to mention
  that MOTD is a file-like object.
* doc/guix.texi (Base Services): Adjust 'mingetty-service' documentation
  accordingly.
* gnu/system/install.scm (installation-services): Adjust accordingly.
  (log-to-info): Use 'program-file' instead of 'gexp->script'.