donate emacsclient utilities to new with-editor.el

Move the macros `magit-with-editor' and `magit-with-git-editor,
the option `magit-emacsclient-executable', and the advice
to `server-switch-buffer' to the new library `with-editor.el'
which is now part of the `git-modes' repository.  The symbols
are renamed accordingly but otherwise unchanged.
Merge branch 'n/with-editor-1' into next