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

Because we cannot yet bring a more recent musl to the mips and mipsel
architectures, we disable support for these constant until a cascading
update with rust-lang/rust has been done.