InputStream::Read() and InputStream::Seek() are called with the mutex
locked.  That means the implementation must not block, or unlock the
mutex before calling into blocking code.

Previously, a slow CD drive could stall the whole MPD process,
including the main thread, due to this problem.

Closes #149

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

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

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.