filter: Implement an in-list() filter

The in-list filter allows us to do simple file-based black- or
whitelisting. The syntax is:

 filter f_whitelist { in-list("/path/to/file.list", value("PROGRAM")); };

Implementing a blacklist is as simple as negating the in-list() filter.

The filter works by pulling in the whole file (one entry per line) and
putting it into a tree structure, and comparing against that on eval.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>