When creating the necessary HOMEBREW_* variables ensure that they aren't
set if their value would be empty.

- only document HOMEBREW_* variables in the manpage (although still
  read from all the original environment variables).
- resort manpage environment variables in alphabetical order
- check the original path for trailing slashes

These are set as `HOMEBREW_BINTRAY_*` and then used by the rest of
`brew` as normal.

Fix those that can be done so without tearing Homebrew to pieces and
remove the comments for those that can never be done.

For many people `brew edit` makes use of the `EDITOR` variable to pick a
sensible editor. With environment filtering enabled unless this editor
is found in the default system PATH it'll fall back to e.g. `vim`.

Instead, ensure that we export the original, pre-filtering `PATH` as
`HOMEBREW_PATH` and use that internally to locate the editor. In future
this same approach will likely be used for requirements to be able to
find tools, too, and for other variables which we want to expose to
Homebrew itself but not other build tools.

Note that `HOMEBREW_PATH` is the same as `PATH` when build filtering
hasn't been enabled.