quotacheck: comment bare literal arguments

docs/CODING_STYLE.md asks for a /* name= */ comment whenever NULL or
another value meaning "unset" is passed to a function. Add it wherever
a bare NULL, 0, false or true is passed for a parameter with a telling
name. Mechanical, generated by tools/check-argument-comments.py --fix.

src/quotacheck/quotacheck.c

@@ -85,7 +85,7 @@ static int run(int argc, char *argv[]) {
 
         umask(0022);
 
-        r = proc_cmdline_parse(parse_proc_cmdline_item, NULL, 0);
+        r = proc_cmdline_parse(parse_proc_cmdline_item, /* userdata= */ NULL, /* flags= */ 0);
         if (r < 0)
                 log_warning_errno(r, "Failed to parse kernel command line, ignoring: %m");