Turns out ucol_getSortKey() does not what I thought it does.

Fixes the broken "search" command.

The attribute must be 0..100 and not 0..1024.  Previously, the code
was inconsistent.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

This case is handled already by playlist_open_in_playlist_dir() (via
playlist_mapper_open()).  And the call didn't work anyway.

Eliminates some overhead and some duplicate code, and fixes a serious
bug: the old code did not append the ".m3u" suffix, and thus the
"load" command was completely broken for stored playlists.  D'oh!

Fixes assertion failure.

The string "base" is understood by locate_parse_type(), but not by
listAllUniqueTags().  The special tag type LOCATE_TAG_BASE_TYPE causes
a crash in PrintUniqueTag().