services: file-systems: Include 'user-file-systems' service.

services: file-systems: Include 'user-file-systems' service.

Previously the KNOWN-FS value used in 'essential-services' would be
incomplete: it would lack all the file systems provided by services that
extend 'file-system-service-type' (/sys/fs/cgroup,
/proc/sys/fs/binfmt_misc, etc.)  Consequently, upon shutdown,
'user-processes' would unmount these file systems before their
corresponding service had been stopped; when their corresponding (e.g.,
'file-system-/proc/sys/fs/binfmt_misc') was stopped, its 'umount' call
would fail.

This was harmless in practice, but this patch makes sure things work as
intended and file systems are unmounted in the right order.

* gnu/services/base.scm (file-system-shepherd-services): Instantiate
'user-file-systems' Shepherd service from here.
(user-unmount-service-type, user-unmount-service): Remove.
* gnu/system.scm (essential-services): Remove call to 'user-unmount-service'.
* gnu/system/install.scm (cow-store-service-type): Adjust comment.