* gnu/system/pam.scm (unix-pam-service): Add pam-entry for "pam_rootok.so" to
auth field when ALLOW-ROOT? is #T.
(base-pam-services): Allow root to run "su" without authentication.
* gnu/system/pam.scm (<pam-limits-entry>): New record type.
(pam-limits-entry, pam-limits-entry->string): New procedures.
* gnu/services/base.scm (pam-limits-service-type): New variable.
(pam-limits-service): New procedure.
* doc/guix.texi (Base Services): Document it.
* 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>.
