path_approx: Apply restrictions for command word

[danielshahaf]

(Spun off from #148)

When the word being completed is the command word, path_approx should only glob things that could be a valid command word: only absolute paths and things in $PATH, which are either executable or (if AUTO_CD is set) directories.

[danielshahaf]

path_approx was added by @Valodim in #115, alongside path_prefix. The latter has a clear use-case, but I honestly don't see why highlighting things that are similar to existing paths, but are not themselves valid paths, is useful at all. (By comparison, we don't highlight misspelled commands — for example, ifcofnig is red — and it wouldn't be useful to do so.)

To be honest, my hunch is that path_approx should simply be dropped, or at least made optional and disabled by default. But I'm open to the possibility that I'm missing some important use-case, in which case, I would love to be enlightened.

[Valodim]

Hm. path_approx was one of these "hey that'd be cool maybe?" features. The use case is mild reassurance (i.e. other than a non-highlighted parameter) that a file does not already exist but a similar one does, which is usually a very distinct scenario from working with an actually existing file name. I admit it's not super important to have in any particular situation, but it has never annoyed me either.

I don't mind either way, feel free to keep or ditch it :)