netlink: refactor KPI for generic Netlink modules

Now that the family and group are completely private to netlink_generic.c,
provide a simple and robust KPI, that would require very simple guarantees
from both KPI and the module:

• Strings are used only for family and group registration, that return ID: uint16_t genl_register_family(const char *name, ... uint32_t genl_register_group(uint16_t family, const char *name, ...
• Once created families and groups are guaranteed to not disappear and be addressable by their ID.
• All subsequent calls, including deregistration shall use ID.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D48845