file-systems: 'mount-file-system' now takes a <file-system> object.

file-systems: 'mount-file-system' now takes a <file-system> object.

* gnu/build/file-systems.scm (mount-file-system): Rename 'spec' to 'fs'
and assume it's a <file-system>.
* gnu/build/linux-boot.scm (boot-system): Assume MOUNTS is a list of
<file-system> and adjust accordingly.
* gnu/build/linux-container.scm (mount-file-systems): Remove
'file-system->spec' call.
* gnu/services/base.scm (file-system-shepherd-service): Add
'spec->file-system' call.  Add (gnu system file-systems) to 'modules'.
* gnu/system/linux-initrd.scm (raw-initrd): Use (gnu system
file-systems).  Add 'spec->file-system' call for #:mounts.