more usage reporting on `apt missing` & no params
missing(): usage and comments

Exploring wether this function actually does anything worthwhile now,
and attempting to divine original intent.

Also moved "x.has_key" pattern to "X in Y", as per
http://stackoverflow.com/questions/1323410/has-key-or-in
prototype missing() does all packages

...not just named package. get_missing(pkg) is indeed just for sussing
out a single package, it should be the _command_ missing that takes
care testing all installed packages. Also I'm not sure the testing for
most current version of installed has any place in this function.
missing command pretty much complete (and useful!)

Remove no longer relevant comments and code, and more nicely report
results. (still not sure about "warning: x is missing", but don't have
good alternative yet).

Still some clean up needed wrt to version currency testing, but need to
think about whether it should be moved or just deleted.