Functions flow_check_cf_bmk_values, flow_check_cf_value_length,
flow4_validate_cf and flow6_validate_cf are now not built with lib but
with conf to enable for better semantic separation.
When writing flow4 { dst 2001:db8::dead:beef/128; }, BIRD crashed on an
not-well-debuggable segfault as it tried to copy the whole 128-bit
prefix into an IPv4-sized memory.