lxc-create: use posix shell instead of bash

lxc-create: use posix shell instead of bash

- use '[ -x /path/prog ]' instead of 'type /path/prog'
- avoid getopt --longoptions
- add \ at after && and || when those are at end of line
- make sure condition expands to empty string if variable is empty

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>