sd-netlink: drop sd_genl_family_t and introduce GenericNetlinkFamily

sd-netlink: drop sd_genl_family_t and introduce GenericNetlinkFamily

Kernel manages each genl family by its name, e.g. "nlctrl" or WG_GENL_NAME,
and its ID (used for nlmsg_type) is determined dynamically when the
corresponding module is loaded.

This commit makes sd-netlink follow the same way; now, sd_genl_family_t
is dropped, and sd_genl_message_new() takes a genl family name. Each
genl family is resolved when it is used first time, and its information
is stored in GenericNetlinkFamily.