Adding GNU/kFreeBSD support. As requested here:
http://code.google.com/p/leveldb/issues/detail?id=38

Use uint64_t instead of size_t in MemEnvTest. As pointed out at
http://code.google.com/p/leveldb/issues/detail?id=41

Fix GCC -Wshadow warnings in LevelDB's public header files,
reported by Dustin.

Add in-memory Env implementation (helpers/memenv/*).
This enables users to create LevelDB databases in-memory.

Initialize ShardedLRUCache::last_id_ to zero.
This fixes a Valgrind warning.

(Also delete port/sha1_* which were removed upstream some time ago.)