The way we currently handle @-versioned formulae linking is pretty
labourius:
- it requires extensive use of `link_overwrite` to avoid the `link`
stage failing on certain install/upgrade scenarios
- we teach people to use `brew link --force` whenever they wish to
link a versioned formulae when it's pretty obvious what's expected
in that situation
Instead, let's:
- automatically unlink other versioned formulae when linking a
versioned formula (either through `brew link` or `install`/`upgrade`
/`reinstall`)
- notify the user what we've done (with the same messaging as if
they had run `brew link` manually)
