system: pam: 'pam-root-service-type' can be extended with transformations.

* gnu/system/pam.scm (<pam-configuration>): New record type.
(/etc-entry): Change 'services' parameter' to 'config'.  Honor the
'transform' field of CONFIG.
(extend-configuration): New procedure.
(pam-root-service-type): Use EXTEND-CONFIGURATION as the 'extend'
field.
(pam-root-service): Add #:transform parameter.  Service value is a
<pam-configuration>.