magit-emacsclient-executable on osx

OSX has an insanely old emacs on it, which no one should ever use. So,
emacs is probably coming from /usr/local via homebrew or something. By
default on OSX, change magit-emacsclient-executable to that one, so it
has the same version as the running emacs from there, and can actually
work. Then, magit will be able to edit commit messages and things like
that.

Without this, magit breaks in depressing and inscrutable ways.