Implement an (in-memory) Avatar cache - reducing the number of disk accesses

when using features that support avatars (e.g. reading msgs, listing files,
listing BBSes in the BBS list).
When used in the terminal server, the cache is located in bbs.mods.avatar_cache
otherwise it's located in the scope of the load()'d library. Applications must
use the lib's read() method to take advantage of the cache. The other lower
level functions (e.g. read_localuser, read_netuser) by-pass the cache on read
but do update the cache with the result. So, generally, avatars should be only
loaded from disk one time during a session/logon.