gnu: Add rsync service.

* doc/guix.texi (Networking Services): Add rsync service documentation.
* gnu/services/rsync.scm (<rsync-configuration>): New file.
* gnu/tests/rsync.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add new files.

Signed-off-by: Christopher Baines <mail@cbaines.net>