Updated default CanonicalizeIP output for IPv6 input. bnc837517

When CanonicalizeIP is invoked with IPv6 input it returns empty NETMASK.
NETMASKs are not used in IPv6 world that much. Moreover
Netmask::FromBits uses IPv4 specific implementation.
Modified Netmask::FromBits for longer prefixes

Do not try to handle non IPv4 prefixes.
Updated changelog.

Conflicts:
	VERSION
	package/yast2.changes