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

[Drew Adams]

Also, you said:

> But I agree that more straightforward customization would be
> better with a clear value of the customizable variable.

I agree about the usefulness of having a user option for this. But
here is one more consideration about that.

Personally, I have no problem in general with commands that bind
user options for their duration (and even with commands that set
user option values), as long as their doc strings tell users that they
do this.

But I think it is vanilla-Emacs practice to forbid such programmatic
changes of option values (even just during a command).

If so, then please also allow also for programmatic use. Code
should be able to bind a variable (non-option, if binding an
option is verboten) to control whether lazy highlighting is
full-buffer.

That is, I believe that Isearch has several cases where there are
both a user option and a non-option variable to control some
behavior. That should be the case for full-buffer highlighting too.