make auto-compression-mode on by default

[Dan Jacobson]

Just noticed that
$ emacs somefile.gz
doesn't gunzip the file for viewing or editing by default.
Maybe it should be the default behaviour.
Else one just looks at a jumble.

That is, (auto-compression-mode) should be on by default, and off only
if made so in .emacs.  Revise the info page to mention this too then.

Else maintainers of multiple machines won't be able to use vanilla
emacs to read the tons of .gz files, without needing to also install a
.emacs.

I mean only one in 1000 users would have need to see raw .gz file guts.