unixsock plugin: Use `src/utils_cmd_listval.[ch]' and remove the local cache.

Since no command uses the cache in the unixsock plugin anymore, it can be
removed. The implementation in `src/utils_cache.[ch]' is now used for all
commands, and all commands are in separate modules. This should be a
performance gain, since the implementation of unixsock used a linked list,
which is much slower than the AVL tree used in the global cache.

Also this resolves a nasty bug: The unixsock plugin used to use the _local_
interval setting when removing values from the cache. The global cache uses the
interval setting of the values themselves, to that mixing different intervals
in a big setup is now compatible with the `listval' and other commands.