dissect: use pager for --help

This output is already too long to fit on an normal terminal, and the
interesting parts are towards the top.

src/dissect/dissect.c

@@ -96,6 +96,8 @@ static int help(void) {
         _cleanup_free_ char *link = NULL;
         int r;
 
+        pager_open(arg_pager_flags);
+
         r = terminal_urlify_man("systemd-dissect", "1", &link);
         if (r < 0)
                 return log_oom();