formula_installer: warn on bottles being skipped.

We want to notify users that non-/usr/local installs may result in some
bottles being not used.

References #18540
Add optional post_install method to Formula.

Sometimes we may want to run commands after bottle installation (such as
creating directories outside the Cellar) so this method allows us to do
so.

Closes #18382.
mysql: fix creation of var/mysql when pouring.