clipp/ironbee.cpp: Remove unused header.
modules/write_clipp.cpp: Use IronBee++ action API rather than C API.
util/field.c: Improve warning message for optimizer with gcc 4.4.
Link libclipp against libmodpbase64.
predicate/ibmod_predicate.cpp: Minor documentation cleanup.
predicate/dot2.cpp: Use sexpr for values rather than nested tables (see below).

Nested tables did have the advantage of using more vertical space and less horizontal space, but were terrible for nested lists.
predicate/ibmod_predicate.cpp: PredicateTrace now optionally takes rule ids as a filter on what to trace.  RNS-1052
modules/header_order: Add header_order module. RNS-679
engine/var.c: Represent a NULL value as "" rather than "NULL" in expansions.
modules/tests/tc_header_order.rb: Add test of repeated header values.