Eliminates some duplicate code, and as a side effect, this works
around clang 3.8 compiler warning because a negative value was
shifted.

This was lost in commit 8ead8f7e, because this was never implemented
for the new API.

Allow the compiler to invalidate buffer contents.

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

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

Replaces GLib's GError.

This old version has been obsolete for many years - time to remove it.

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

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

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.