Add a in-memory cache

Add a in-memory cache

This is a more general API that caches data with a LRU scheme. See
include/cache.h. No comments yet, I'm still working on it. But Jeremy has given
me a hint in one of his checkins that he would like to make use of this now.

The idea is that we get rid of all our silly little caches and merge them all
into one cache that we can then very easily trim, for example even with a
smbcontrol message if someone decides memory is tight. The main user is the
stat cache, this patch also converts the getwd cache. More caches to come.