Forbid TODO in tidy

Forbid TODO as either a sneaky or an accidental way of evading the
requirement to annotate FIXMEs with issue numbers.

Of course, there are many other ways to evade this requirement,
but one should draw the line somewhere...

src/etc/tidy.py

@@ -28,6 +28,8 @@ try:
             if line.find("FIXME") != -1:
                 if re.search("FIXME.*#\d+", line) == None:
                     report_err("FIXME without issue number")
+            if line.find("TODO") != -1:
+                report_err("TODO is deprecated; use FIXME")
         if (line.find('\t') != -1 and
             fileinput.filename().find("Makefile") == -1):
             report_err("tab character")