lvmsnap.sh: fix errors in getopts

* Remove unused options from list

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>
lvmsnap.sh: add genuine error mesages

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>
lvmsnap.sh: streamline check_root function

Signed-off-by: Andrew Turner <andrewjamesturner0@gmail.com>