adds new labelmap api to create a name <-> bit mapping
from a text file (default: /etc/xtables/connlabel.conf).
nfct_labelmap_new(filename) is used to create the map,
nfct_labelmap_destroy() releases the resources allocated for the map.
Two functions are added to make map lookups:
nfct_labelmap_get_name(map, bit) returns the name of a bit,
nfct_labelmap_get_bit returns the bit associated with a name.
The connlabel attribute is represented by a nfct_bitmask object, the
nfct_bitmask api can be used to test/set/get individual bits
The exisiting nfct_attr_get/set interfaces can be used to read or
replace the existing labels associated with a conntrack with a new set.
Signed-off-by: Florian Westphal <email@example.com>