This reverts commit a577944a.

Will be replaced by new options for the old commands "search" and
"find".

Implements Mantis ticket 3843.

Check was the wrong way around, and made all valid values invisible.

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Replaces GLib's g_ascii_strcasecmp().