services: auditd: Provide default configuration directory.

services: auditd: Provide default configuration directory.

* gnu/services/auditd.scm (auditd.conf)
(%default-auditd-configuration-directory): New variables.
(<auditd-configuration>): Switch to 'define-record-type*'.
[configuration-directory]: New field.
(auditd-shepherd-service): Honor 'configuration-directory'.  Pass #:pid-file.
(auditd-service-type)[description]: Tweak.
[default-value]: Provide 'configuration-directory'.
* doc/guix.texi (Miscellaneous Services): Update docs to reflect
changes.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>