Add `IFF_*` constants  to linux, android and fuchsia

Add constants to android and fuchsia:

*    IFF_TUN
*    IFF_TAP
*    IFF_NO_PI

Type problem: https://github.com/rust-lang/libc/issues/915

so change constants type `libc::c_short` to `libc::c_int` :

*    IFF_*

add some sysv shared memory to macosx

Missing linux intrinsics

Add netfilter constants to Linux

Constants and enums from [`netfilter.h`] and [`nf_tables.h`].

I was not really sure what to do with the enums, for example `nf_tables_msg_types`. But since I could not find a C enum mapped into a Rust enum in this crate I assumed you wanted them as constants instead.

I also wanted to get the `nf_inet_addr` union in here. But I did not find any unions in this crate so I was not sure how you would prefer it represented.

There are of course a billion more netfilter constants and types to map. But this is a good start.

[`netfilter.h`]: https://github.com/torvalds/linux/blob/ead751507de86d90fa250431e9990a8b881f713c/include/uapi/linux/netfilter.h
[`nf_tables.h`]: https://github.com/torvalds/linux/blob/ead751507de86d90fa250431e9990a8b881f713c/include/uapi/linux/netfilter/nf_tables.h

Add posix_spawn bindings

Do not assume dynamic linking in musl/mips targets

Add missing FreeBSD APIs

Process descriptors, realtime process priority, a couple missing kqueue event filters.

Add syscall table for musl/powerpc64

CI still runs version 10

Available since FreeBSD 9.0, this API allows using local descriptors
instead of global PIDs. It works in Capsicum sandboxes.

add DT_UNKNOWN

The `d_type` field of `struct dirent` can be `DT_UNKNOWN` if a
filesystem doesn't support returning the file type in directory entries.
Linux's readdir(3) man page, for example, says: "Currently, only some
filesystems [...] have full support for returning the file type in
d_type. All applications must properly handle a return of DT_UNKNOWN."

The `d_type` field of `struct dirent` can be `DT_UNKNOWN` if a
filesystem doesn't support returning the file type in directory entries.
Linux's readdir(3) man page, for example, says: "Currently, only some
filesystems [...] have full support for returning the file type in
d_type. All applications must properly handle a return of DT_UNKNOWN."

Bsd domainname

Closes #868 .

Fix containers using ubuntu 17.04

Apparently 17.04 has gone away? Or updates are failing now?

Looks like ubuntu 17.04 is broken now and it fails to link on 17.10 and 18.04.
It seems to work on stretch though!

Apparently 17.04 is broken now?

Closes #868 .

Closes #901 .

Closes #900 .

Closes #899.

Closes #874 .

Add sockaddr_ctl on Mac