Update the struct attributes, important for facades like the "rewind"
plugin.  To replace buffer().

This is not necessary since all relevant input plugins have been moved
to the I/O thread, and there is no remaining useful buffer()
implementation.  This also fixes a busy loop when playing radio.

Needed for the I/O thread.

Use it in handle_lsinfo(), and eliminate some duplicate code.

Add nonnull attributes and fix all callers.

This is specific to gcc, not to mpd.

New db_get_song() implementation.

Let the caller decide if the current directory should be visited.

.. to recurse.  A visitor may not have one, but still be interested in
a recursive search.

Use this struct for db_walk().

Improve error reporting and handling.  command.c gets the new function
print_error(), which sends a GError to the client.

Reduce header dependencies.

Only used there.

First draft, not really pluggable currently - hard-coded to use the
"simple" plugin, and calls several of its internal functions.

The API is very simple currently, all searches are still performed
over the root "directory" object.  Future changes to the API will move
those search implementations into the plugin, to allow more efficient
implementations, or implementations that don't have the whole tree in
memory all the time.

For TAG_NUM_OF_ITEM_TYPES.

No writers.

Better error messages.

Remove the slash hack, allocate memory for the user name.

Just advance the "path" pointer.

config_get_path() was somewhat flawed, because it pretended to be a
function, when it really had a side effect.  The second flaw was that
it did not return the parser error, instead it aborted the whole
process, which is bad style.  The new function returns a duplicated
(modified) string that must be freed by the caller, and returns a
GError on failure.

No caller needs to write.

config_add_block_param() cannot fail, which makes it easier to use.

Reduce indent.