services: guix: Add 'log-file' configuration option.

* gnu/services/base.scm (<guix-configuration>)[log-file]: New field.
(guix-shepherd-service): Pass #:log-file to 'make-forkexec-constructor'.
* gnu/services/admin.scm (simple-rotation-config): Take a list of
files and join them with commas.
(%default-rotations): Add /var/log/guix-daemon.log.
* doc/guix.texi (Base Services): Document it.