network.subr: improve configuration of cloned gif(4) interfaces

ifconfig(8) syntax allows to specify only single address_family,
so we need additional invocation of ifconfig to support configuration
of cloned gif interface that may use different address families
for its internal and external addresses.

Also, ifconfig(8) does not allow to omit "inet6" keyword for address family
specifying IPv6 addresses as outer addresses of the interface.

Also, address_family is not "parameter" and it has to go before parameters
including "tunnel" keyword, so "ifconfig gif0 tunnel inet6 $oip1 $oip2" would be
wrong syntax and only "ifconfig gif0 inet6 tunnel $oip1 $oip2" is right.

With this change, the following works:

gifconfig_gif0="inet6 2a00::1 2a01::1"
ifconfig_gif0="inet 10.0.0.1 10.0.0.2 netmask 255.255.255.252"

MFC after: 2 weeks