conf: introduce WithDefault function

This patch introduces a new set of functions to the ConfGetChildValue
family. They permit to look under a default node if looking under
base node as failed. This will be used to access to default parameters
for a data type (for instance, first usage will be interface).
af-packet: add support for 'default' interface

This patch adds support for 'default' interface which is used to get
parameter values when per-interface is not defined.
pfring: add support for 'default' interface
pcap: add support for 'default' interface
conf: add unittest for WithDefault functions.
Adds support for the geoip keyword

Adds support for match-on conditions (src, dst, any, both)
Uses GEOIP_MEMORY_CACHE for performance reasons
Adds support for negation and multiple countries in the same rule

Bug fixes

Changed to take flow direction from rule, if present

Comments addressed. Unit tests added.