formula.bottle is nil when bottle.compatible_cellar? is false.
Use formula.bottle_specification.compatible_cellar? rather
than formula.bottle.compatible_cellar?.
Treat a `brew install` command as normal i.e. link by default unless
keg-only and only specify whether a keg should be linked when upgrading
or reinstalling. Also, adjust the naming accordingly so it's more
obvious that this is the case.
This means if a user has manually `brew unlink` or `brew link --force`d
something then that status will be preserved after they `brew upgrade`
or `brew reinstall` that formula.
This generally should make things that are keg-only by default easier
to swallow.
