Fixes -Winconsistent-missing-override (clang 3.6).

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Replaces GLib's g_str_has_prefix().

This was a memory leak, because "mime" was a std::string which created
another copy and discarded the allocated buffer.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Omitting it has the same effect as returning false unconditionally.

Eliminate input_stream_init() and input_stream_deinit().

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

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

input_stream_buffer() has a check for "buffer==NULL", so we don't need
an explicit empty implementation.

All close() implementations must call this method.