change naming convention for the lru "save history" cache, so that

change naming convention for the lru "save history" cache, so that
ports like "lang/chicken/core" do generate files like lang.chicken.core.lru
instead of lang.chicken.core (which can create confusion in people's mind)

do so transparently by reading the old file if need be, and removing it
afterwards.

Funny thing noticed by tb@

ok tb@, sthen@