Reimplementation of prefix sets.

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.

lib/bitops.h

@@ -17,3 +17,5 @@
 
 u32 u32_mkmask(unsigned n);
 int u32_masklen(u32 x);
+
+u32 u32_log2(u32 v);