services: Make a single extensible 'file-systems' service.

services: Make a single extensible 'file-systems' service.

Previously we would create one 'file-system-service-type' instead per
file system.  Now, we create only one instance for all the file
systems.

* gnu/services/base.scm (fstab-service-type)[compose]: Change to
CONCATENATE.
(file-system-shepherd-service): Change to return either one
<shepherd-service> or #f.
(file-system-service-type): Pluralize 'name'.  Adjust
SHEPHERD-ROOT-SERVICE-TYPE extension to above changes.  Add 'compose'
and 'extend'.
(file-system-service): Remove.
* gnu/system.scm (other-file-system-services): Rename to...
(non-boot-file-system-service): ... this.  Change to return a single
FILE-SYSTEM-SERVICE-TYPE instance.
(essential-services): Adjust accordingly.