SLB: fwmark config generating enhancement

Ability to generate multiple 'virtual_server fwmark XXX' sections
for each IP family, if RS pool contains both v4 and v6 real servers.

Also, do not generate virtual_server section without active real servers
API change: constructIPRange and IPNetworkCmp

constructIPRange: default mask is the maximum possible mask for an
address family
IPNetworkCmp: new return value '2' to make this function symmetric