When playing finite http streams, e.g. something from soundcloud,
eventloop stalls after pausing writefunc. TimeoutMonitor is cancelled
by the time resume happens, so when enough data is consumed writefunc
is called only once. Calling InvalidateSockets() from
input_curl_resume() seems to fix the issue.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Omitting it has the same effect as returning false unconditionally.

Handle timeout internally.

Simplify the API, don't use GLib specific integer type.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

This plugin is cumbersome to support, now that MPD is migrating away
from GLib and the GLib event loop.  It has no practical advantages
over the CURL plugin.  Soup requires the bloated GType library.

Eliminate input_stream_init() and input_stream_deinit().

This is the version present in Ubuntu Oneiric, the oldest distribution
with gcc 4.6.  Debian Squeeze is off target, because it has gcc 4.4,
which is unable to compile MPD anyway.

This commit drops all API compatibility hacks for older versions.

Hide the definition from C code, to prepare the transition to C++.