Switch to Fundamental mode by killing current buffer's local variables.
Most local variable bindings are eliminated so that the default values
become effective once more. Also, the syntax table is set from
`standard-syntax-table', the local keymap is set to nil,
and the abbrev table from `fundamental-mode-abbrev-table'.
This function also forces redisplay of the mode line.
Every function to select a new major mode starts by
calling this function.
As a special exception, local variables whose names have
a non-nil `permanent-local' property are not eliminated by this function.
The first thing this function does is run
the normal hook `change-major-mode-hook'.
(fn)