This removes the insertion offset parameter to playlist_Add(),
playlist_AddExt() and playlist_AddInput().

intf_InsertItem() was the only occurrence where the offset was zero,
a side effect of how the command line is parsed backward. This is now
done explicitly with playlist_NodeAddInput().

A non-zero positive offset made no sense, since:
 - the number of children of a node can change asynchronously while the
   playlist lock,
 - the back-end asserted or triggered undefined behaviour if the offset
   was out of range.
DBus was the only occurrence of this bug (see also #17451 comment:4),
and has been fixed in an earlier change.

In all remaining call sites, the offset was PLAYLIST_END. This removes
the parameter which would otherwise constitute a useless constant.