For type safety and code readability.

Eliminate duplicate definition (in input plugin and decoder plugin).

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

Fixes potential nullptr dereference.

Fixes potential nullptr dereference.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

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

Replaces GLib's GError.

FfmpegDecoderPlugin: Allocate the interleaved buffer as needed instead of using an arbitrary big constant.

Also, save a memcpy when FFmpeg outputs interleaved audio.

Fixes build with FFmpeg 2.0.

Now that even Debian stable has picked up version 0.8, we can safely
make this a requirement, and remove a lot of old cruft.