Instead of caching struct passwd and struct group in the red-black

Instead of caching struct passwd and struct group in the red-black
tree, store a struct cache_item which includes both the key and
datum.  This allows us to user the actual name that was looked up
as the key instead of the contents of struct passwd or struct group.
This matters because the name in the database may not match what
we looked up, due either to case folding or truncation (historically
at 8 characters).  Also mark the disabled calls to sudo_freepwcache()
and sudo_freegrcache() as broken since we use cached data for things
like set_perms() and the logging functions.  Fixing this would
require making a copy of the structs for user and runas or adding
a reference count (better).

--HG--
branch : 1.7