Toggle Master mode.
With a prefix argument ARG, enable Master mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.
When Master mode is enabled, you can scroll the slave buffer
using the following commands:
Uses keymap `master-mode-map', which is not currently defined.
The slave buffer is stored in the buffer-local variable `master-of'.
You can set this variable using `master-set-slave'. You can show
yourself the value of `master-of' by calling `master-show-slave'.
(fn &optional ARG)