installer: Add JFS support.

* gnu/installer/newt/partition.scm (run-fs-type-page): Add ‘jfs’ to the
list box.
* gnu/installer/parted.scm (user-fs-type-name, user-fs-type->mount-type)
(partition-filesystem-user-type): Add ‘jfs’ mapping
(create-jfs-file-system): New procedure.
(format-user-partitions): Use it.
* gnu/installer.scm (set-installer-path): Add jfsutils.