GME starts all track indexes at zero, but subtune prefixes
start at one. This fixes an off-by-one error during track
enumeration.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Broken by commit 0abee77e

Got lost in 0abee77e

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Fixes http://bugs.musicpd.org/view.php?id=4432

Use simple string and path parsing functions instead of GLib's
g_pattern_match(), which was used in a very clumsy way.

There is duplicate code in the two.

For type safety and code readability.