Fixes deadlock on small responses.

While seeking, metadata must not be updated.  ResponseBoundary() was
added in MPD 0.19.1, but I forgot to add the IsSeeking() check there.
This caused the "seekable" flag to reset.

Fixes playback of redirected streams.

The second time init was called, http_200_aliases pointed to a freed pointer
and leaded to a SEGFAULT.

New base class for other InputStream implementations that run in the
I/O thread.

Fixes assertion failure.

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.

Replaces its own weird buffering code.

Code simplification.

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

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.