Constants for the linux nfqueue netlink protocol (userspace firewall).

Continuation of #1562 and #1571.

This triggers a warning on a recent nightly, which in turn breaks CI due to
`#![deny(warnings)]` in libc-test/build.rs

Only Netlink interface link attributes up IFLA_STATS are currently
available. This commit adds IFLA_COST to IFLA_PROTO_DOWN.

See https://github.com/torvalds/linux/blob/54ecb8f7/include/uapi/linux/if_link.h#L106

Signed-off-by: Arthur Gautier <baloo@gandi.net>

First batch for #1562

See
https://github.com/torvalds/linux/blob/54ecb8f7/include/uapi/linux/if_link.h#L354-L362

It's *msg_prio* in both manpages and posix.

Closes #659

Add struct sockaddr_vm and VMADDR_ constants to use
VSOCK on Linux. VSOCK is present since Linux 3.9

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

This was added in Linux 5.1 and will only show up in the next glibc
release, thus skip in tests.

The `pad` or `__reserved`  fields are not always 0 on some platforms,
so when used in the `PartialEq` implementation being used, fails some
comparisons. This commit manually implements the extra traits to
correct this behavior.

These were originally added as c_short types, but all other IFF_ constants
are c_int. This commit changes them to match all the other constants for
consistency.

Non-existent constants like SYS_syscalls have been removed