nat: NAT44 ED improvements and fixes

Fixed FIB entry per interface, pool and
static mapping registrations in NAT control plane.
Improved FIB configuration handling in NAT -
added functions to generalize handling of FIB & NAT,
cleaned up interface callback functions. Changed "is_"
macros to inline functions for easier debugging.

Type: improvement

Change-Id: I68a17761a975dd819139ae94e52a8a85d3f27ffc
Signed-off-by: Filip Varga <fivarga@cisco.com>