Robustify netlink response parsers

Robustify netlink response parsers

* socketutils.c (inet_parse_response, unix_parse_response): Change
return type from bool to int, return -1 on all parse errors except
inode mismatch.
(receive_responses): Stop on the first nlmsg_type that is not
SOCK_DIAG_BY_FAMILY, also stop when the parser returns -1.
* tests/netlink_inet_diag.c (check_responses): Stop on short messages,
on first nlmsg_type that is not SOCK_DIAG_BY_FAMILY, print more verbose
diagnostics for NLMSG_ERROR.
* tests/netlink_unix_diag.c (check_responses): Likewise.