netlink: underscore snl_get_genl_family_info() to discourage its use

This function uses already supposedly opaque struct _getfamily_attrs as
the argument and it fills it with pointers to volatile memory, which makes
it is unsafe for general use. While here also underscore structures that
hang off the struct _getfamily_attrs.

Small programs like powerd(8) and RPC daemons are converted to use
snl_get_genl_mcast_group() and/or snl_get_genl_family(). The genl(1)
utility was fixed not to mix its own parsers with parsers declared in
netlink_snl_generic.h.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D48480