tree-wide: add clickable man page link to all --help texts

This is a bit like the info link in most of GNU's --help texts, but we
don't do info but man pages, and we make them properly clickable on
terminal supporting that, because awesome.

I think it's generally advisable to link up our (brief) --help texts and
our (more comprehensive) man pages a bit, so this should be an easy and
straight-forward way to do it.

src/busctl/busctl.c

@@ -1726,6 +1726,13 @@ static int set_property(int argc, char **argv, void *userdata) {
 }
 
 static int help(void) {
+        _cleanup_free_ char *link = NULL;
+        int r;
+
+        r = terminal_urlify_man("busctl", "1", &link);
+        if (r < 0)
+                return log_oom();
+
         printf("%s [OPTIONS...] {COMMAND} ...\n\n"
                "Introspect the bus.\n\n"
                "  -h --help               Show this help\n"
@@ -1768,7 +1775,10 @@ static int help(void) {
                "  set-property SERVICE OBJECT INTERFACE PROPERTY SIGNATURE ARGUMENT...\n"
                "                          Set property value\n"
                "  help                    Show this help\n"
-               , program_invocation_short_name);
+               "\nSee the %s for details.\n"
+               , program_invocation_short_name
+               , link
+        );
 
         return 0;
 }