bug#29360: 26.0; Add full-buffer choice for `isearch-lazy-highlight'

[Drew Adams <drew.adams@oracle.com>]

See bug #21092 for the motivation and reasons behind this bug report.
The aim of that bug report was never taken care of, which was to let
you force lazy-highlight highlighting on the full visible portion of
the buffer (where that refers to the full buffer or the current
buffer restriction).  IOW, optionally do not restrict highlighting
to what is currently on screen, as is done now.

The #21092 bug report mistakenly took a `nil' value of
`lazy-highlight-max-at-a-time' as meaning just that - just what the doc
said: act on the full buffer, not just on the text that is currently in
the window.  The true meaning of nil for that variable is just to
highlight all of the search hits visible in the window.

This new bug aims to finally get what was really being requested in
#21092: the ability to cause lazy highlight to act on the full buffer,
not just the text currently visible in the window.

The outcome of #21092 and other bugs led to the creation of variable
`isearch-lazy-highlight'.  At the end of #21092, realizing that closing
#21092 did not, in fact, respond to the aim of #21092, Juri suggested
that a separate report be filed, to request a new possible value for
`isearch-lazy-highlight' which will cause the full buffer to get the
lazy-highlight highlighting.  That is the purpose of this bug report:
please add such a `buffer' or `full-buffer' value.

In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''