netlink: call get_fd_nl_family before nlmsghdr decoding

netlink: call get_fd_nl_family before nlmsghdr decoding

Prepare for decoding of NETLINK_KOBJECT_UEVENT.  Messages of the latter,
unlike traditional netlink families, don't contain a header at all.

* netlink.c (NL_FAMILY_*): Remove enum.
(get_fd_nl_family): Replace NL_FAMILY_ERROR with -1.
(decode_nlmsg_type): Update the comment.
Skip family specific type decoders for type < NLMSG_MIN_TYPE.
(decode_nlmsghdr_with_payload): Skip family specific decoders
for type < NLMSG_MIN_TYPE && type != NLMSG_DONE.
(print_nlmsghdr): Move get_fd_nl_family invocation ...
(decode_nlmsghdr_with_payload): ... here.