add IPv6

Add IPv6 support. Not fully documented. IPv6 support can be enabled in the application makefile by:
XTCP_IPV6 = 1
ifeq ($(XTCP_IPV6),1)
XCC_FLAGS = -O2 -g -DIPV6=1
else
XCC_FLAGS = -O2 -g -DIPV6=0
endif
If nothing (xtcp_ipv6) is defined, then module_xtcp will assume the usual ipv4.