Split up the fake "missing" warning category into an actual category

Ever since the warning for missing printf arguments was added in
v5.11.2-116-g7baa469 the "missing" warning category has been defined in
terms of the "uninitialized" category, so you couldn't turn it on/off
independently of that.

As discussed in RT #121025 I'm hacking on adding a new "reduntant"
category for too many printf arguments. So add the long-missing
"missing" category in preparation for that for consistency.