tools/list: enclose the the various field names in quotes

Because otherwise the 'no' layout is treated as disagreement with whatever is to
be disagreed with. Fixed in YAML 1.2 but that's not universally supported.

Fixes #268

tools/registry-list.c

@@ -159,12 +159,12 @@ main(int argc, char **argv)
         const char *variant = rxkb_layout_get_variant(l);
         const char *brief = rxkb_layout_get_brief(l);
 
-        printf("- layout: %s\n"
-               "  variant: %s\n"
-               "  brief: %s\n"
+        printf("- layout: '%s'\n"
+               "  variant: '%s'\n"
+               "  brief: '%s'\n"
                "  description: %s\n",
                rxkb_layout_get_name(l),
-               variant ? variant : "''",
+               variant ? variant : "",
                brief ? brief : "''",
                rxkb_layout_get_description(l));
 
@@ -173,7 +173,7 @@ main(int argc, char **argv)
         if (iso639) {
             const char *sep = "";
             while (iso639) {
-                printf("%s%s", sep, rxkb_iso639_code_get_code(iso639));
+                printf("%s'%s'", sep, rxkb_iso639_code_get_code(iso639));
                 iso639 = rxkb_iso639_code_next(iso639);
                 sep = ", ";
             }
@@ -184,7 +184,7 @@ main(int argc, char **argv)
         if (iso3166) {
             const char *sep = "";
             while (iso3166) {
-                printf("%s%s", sep, rxkb_iso3166_code_get_code(iso3166));
+                printf("%s'%s'", sep, rxkb_iso3166_code_get_code(iso3166));
                 iso3166 = rxkb_iso3166_code_next(iso3166);
                 sep = ", ";
             }
@@ -199,7 +199,7 @@ main(int argc, char **argv)
     while (g) {
         struct rxkb_option *o;
 
-        printf("- name: %s\n"
+        printf("- name: '%s'\n"
                "  description: %s\n"
                "  allows_multiple: %s\n"
                "  options:\n",