Stop HTTP clients from exploiting MPD via forged POST requests.

For database plugins that don't use the UpdateService.

Manipulates the playback range of a queued song.

Implement a "bulk" edit mode that postpones both UpdateQueuedSong()
and OnModified().  This way, the playlist version gets incremented
only once.  More importantly: when adding multiple songs to a queue
that consists of only one song, the first song that got added will
always be played next.  By postponing this choice, all newly added
songs get a chance to become the next song.  Fixes the second (and
last) part of Mantis ticket 0004005.

Eliminate extra allocations for the std::list node instances.

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

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().

Lists files and directories.  Supports storage plugins.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Add the new idle event to Idle.hxx/Idle.cxx.

Implements the error checks missing in commit 0bad8406