system: Introduce 'file-systems' Shepherd service.

* gnu/services/base.scm (file-system-shepherd-services): New procedure.
(file-system-service-type): Use it as the SHEPHERD-ROOT-SERVICE-TYPE
extension.
(user-processes-service-type): Change to take a single 'grace-delay'
parameter.
(user-processes-service): Remove 'file-systems' parameter.  Pass
GRACE-DELAY as the only value for the service.
* gnu/system.scm (essential-services): Adjust accordingly.