emacs: define a few variables as automatically buffer-local

emacs: define a few variables as automatically buffer-local

Define these variables as automatically buffer-local, meaning that
they always become buffer-local when set unless explicitly told
otherwise using `setq-default' or when using the Custom interface.

Previously they were declared, which keeps the byte-compiler quiet but
is not actually the same as being defined.  `notmuch-search-mode' then
made them buffer-local in the current buffer and then set the local
values.  This works but is not kosher.

The definitions of the three non-option variables have to be moved up
a bit to enable the change in the next commit, which see.