We set buffer->pos to buffer->end in function buffer_print() when
bvsnprintf() failed, so there would be uninitialized memory between
the old buffer->pos and the current buffer->pos.
Prefix sets were broken beyond any repair and have to be reimplemented.
They are reimplemented using a trie with bitmasks in nodes.
There is also change in the interpretation of minus prefix pattern,
but the old interpretation was already inconsistent with
the documentation and broken.
There is also some bugfixes in filter code related to set variables.
