shared/verbs: allow commands to specify footer highlighting

In practice I think this will ever only be used with ansi_highlight_red(),
but it's cleaner to add this generic extension rather than e.g. a boolean
highlight_footer_red.

The options is passed through via command_print_help_full(), not through
the COMMAND structure, which means that commands with 'help' verb are not
covered by this. The red footer highligting is used for legacy commands,
which don't have verbs, so I think this is OK. Making it generic and
covering 'help' verbs would require more refactoring, and I think it
doesn't make sense to add it until it's actually needed.

src/resolve/resolvconf-compat.c

@@ -181,7 +181,7 @@ int resolvconf_parse_argv(int argc, char *argv[]) {
                 OPTION_NAMESPACE("resolvconf"): {}
 
                 OPTION_COMMON_HELP:
-                        return command_print_help_full("resolvconf");
+                        return command_print_help_full("resolvconf", /* footer_ansi_seq= */ NULL);
 
                 OPTION_COMMON_VERSION:
                         return version();

src/resolve/resolvectl.c

@@ -3498,7 +3498,7 @@ static int native_parse_argv(int argc, char *argv[], char ***remaining_args) {
                 OPTION_NAMESPACE("resolvectl"): {}
 
                 OPTION_COMMON_HELP:
-                        return command_print_help_full("resolvectl");
+                        return command_print_help_full("resolvectl", /* footer_ansi_seq= */ NULL);
 
                 OPTION_COMMON_VERSION:
                         return version();