Add IPV6_UNICAST_HOPS to fuchsia module

Recent change #925 added constants for IPV6 multicast, but seems to have inadvertently left off IPV6_UNICAST_HOPS from the fuchsia module.

Fixes #932

Recent change #925 added constants for IPV6 multicast, but seems
to have inadvertently left off IPV6_UNICAST_HOPS from fuchsia.

Fixes #932

Add posix_spawn bindings for FreeBSD

Fix --no-default-features builds

Closes #928 #929 #684

Bump to 0.2.37

This includes:
- more multicast fields
- netfilter/nftables fields

Add netfilter/nftables fields

Adding a few missing `NF_` constants to more complete the story around that header. Then most importantly add a lot of `NFT_` constants needed in my nftables library. Some were added back in #911, but as the library grew I realized I needed more constants.

add IPV6_MULTICAST_IF and IPV6_MULTICAST_HOPS

These were missing from the net2 crate. Adding them here so that the higher functions can be added to net2 later. They are necessary for declaring the interface for ipv6 multicast packets (route), and limits on the number of hops an ipv6 packet can travel

Linux reference: https://github.com/torvalds/linux/blob/master/include/uapi/linux/in6.h#L168-L170
macOS reference: https://github.com/apple/darwin-xnu/blob/master/bsd/netinet6/in6.h#L538-L540

Was able to move some of them up one level so they are valid on more
platforms with less duplication

Add ETH_* constants on Android

Adding ETH_* constants that were found here on Linux but not on Android.

Source Reference is [linux/if_ether.h](https://android.googlesource.com/platform/bionic/+/master/libc/kernel/uapi/linux/if_ether.h) on Google OpenSource.

Thank-you.

/cc nix-rust/nix#865

Add netlink structs to all notbsd platforms

Adding the remaining structs from `<linux/netlink.h>`. Sadly very duplicated, but that's how it ended up when added to all "notbsd" platforms except musl. If there is a better way to structure it to reduce duplication I'm all ears!

Prefer hexadecimal representation of O_TMPFILE on linux-sparc64

While working on the libc bindings for sparc-linux, I stumbled across this typo in the O_TMPFILE definition on linux-sparc64.

Compare with:

> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sparc/bits/fcntl.h;hb=16efad5171ac1ac2c8728405f2703045f08c494b#l42
> https://sources.debian.org/src/glibc/2.26-6/sysdeps/unix/sysv/linux/sparc/bits/fcntl.h/#L42

CC @jrtc27

Add termios ioctl constants to NetBSD

These are all declared as `c_ulong` because that's what BSD's `ioctl()` function is declared with.

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