[cmds] Update 'sys' and 'mknod' to work for updating ELKS

[ghaerr]

Enhances the bin/sys shell script and mknod command to update an existing ELKS volume from the boot volume.

Discussed in #2181 (comment).

@floriangit: I created this PR as there might have been a misunderstanding about the need for set -e/set +e in bin/sys.
(set -e tells the shell to stop execution on an error). This PR fixes sys so that any error will stop the script, except for the makeboot execution, which as you pointed out returns the filesystem type as it's return value. So a set +e is required only before makeboot, and otherwise set -e is always set. Should makeboot encounter an error, it returns 255, and sys will return 1 (error) to its caller. Otherwise the script now will report "Finished successfully." to the user.

mknod was changed to return 0 on successful -p option, as discussed.

Also fixed is "Can't create symlink: File Exists", which is reported by cp -fR when copying a full system and encountering the symlink already created on the destination volume. The fix is to unlink the symlink prior to creating it, when the -f option is in effect.

[floriangit]

very smooth updating now! 🥇