mpv: change install path of mpv.app

Homebrew also changes the shared library identification name of a
dynamic shared library in #{previx}/{bin, lib and Frameworks}, so we
have to change install path of mpv.app to avoid the overwriting.
mpv: fix brew audit complaints
libass-ct: fix audit error
mpv: cosmetic changes

Use Pathname#/ (aliased to join) instead of #+
mpv: fix zsh completion option semantic

Default to building the zsh completions. This was erroneously changed to
defaulting to not building them in commit fc67bcfe0.