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

src/cryptsetup/cryptsetup.c

@@ -111,7 +111,7 @@ static int parse_one_option(const char *option) {
                         return 0;
                 }
 
-        } else if (streq(option, "readonly"))
+        } else if (streq(option, "readonly") || streq(option, "read-only"))
                 opt_readonly = true;
         else if (streq(option, "verify"))
                 opt_verify = true;