Eliminate dependency on io_thread_get().

Fixes the dreaded error "Failed to create : No such file or
directory".

 https://bugs.musicpd.org/view.php?id=4625

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

The old struct config_param remains only for top-level string options.

Supports "~/" expansion.  Forces us to switch from "const char *" to
AllocatedPath, which is a good thing.