Make index file locking code reusable to others.

Make index file locking code reusable to others.

The framework to create lockfiles that are removed at exit is
first used to reliably write the index file, but it is
applicable to other things, so stop calling it "cache_file".

This also rewords a few remaining error message that called the
index file "cache file".

Signed-off-by: Junio C Hamano <junkio@cox.net>