cryptsetup: accept both "read-only" and "readonly" spellings

Mukund Sivaraman pointed out that cryptsetup(5) mentions the "read-only"
option, while the code understands "readonly".

We could just fix the manpage, but for consistency in naming of
multi-word options it would be prettier to have "read-only". So let's
accept both spellings.

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=903463