* gnu/services/base.scm (user-processes-shepherd-service): New
procedure, taken from former 'user-processes-service-type'. Add
REQUIREMENTS argument; remove GRACE-DELAY argument.
(user-processes-service-type): Redefine in terms of 'service-type'.
(user-processes-service): Remove.
(file-system-service-type): Extend USER-PROCESSES-SERVICE-TYPE.
* gnu/system.scm (essential-services): Use USER-PROCESSES-SERVICE-TYPE
directly.