system: Mapped devices needed for boot do not yield Shepherd services.

system: Mapped devices needed for boot do not yield Shepherd services.

Fixes <https://bugs.gnu.org/31889>.
Reported by Taylan Kammer <taylanbayirli@gmail.com>.

* gnu/system.scm (non-boot-file-system-service)[mapped-devices-for-boot]:
New variable.
Remove dependencies of FS that are members of MAPPED-DEVICES-FOR-BOOT.
(mapped-device-user): Rename to...
(mapped-device-users): ... this.  Use 'filter' instead of 'find'.
(operating-system-user-mapped-devices)
(operating-system-boot-mapped-devices): Use 'any
file-system-needed-for-boot?' instead of looking at the first user.
* tests/system.scm ("non-boot-file-system-service"): New test.