Replaces GLib's g_str_has_prefix().

mixramp_delay==nan() causes severe problems with cross-fading.

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

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

Look up top-level config options by enum (= integer), not by name
string.

When set, MPD will not auto-start playback on startup; it will be in
"paused" state.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

An '\n' was erroneously inserted in the line containing the state, e.g.
"state: \nplay" instead of "state: play".

Fix for bug #2992.

Use the functions song_save() and song_load() to use the same format
as in the database file for those songs which need the tags.

The only caller doesn't use its return value, and the value isn't
useful anyway.