shared/options: convert mode boolean to an enum

This will make it easier to add new modes of operation later.
But I'm happy with how this came out — I think the mode setting
is nicer to read then the old bool.

src/nspawn/nspawn.c

@@ -586,7 +586,7 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        OptionParser state = { argc, argv, /* stop_at_first_nonoption= */ true };
+        OptionParser state = { argc, argv, OPTION_PARSER_STOP_AT_FIRST_NONOPTION };
         const Option *opt;
         const char *arg;