Auto merge of #1551 - senden9:fixes/uclibc_net_const, r=gnzlbg

Add networking and futex constants for uclibc

I needed this variables to run some rust program that uses `net2` on a embedded Linux platform.
This platform has as MIPS 74Kc CPU and uses uclibc.

"Proof" that this variables are the same under all uclibc targets:
<details>
  <summary>IP constants</summary>

  ```
➜  uClibc git:(master) git remote -v
origin	git@github.com:kraj/uClibc.git (fetch)
origin	git@github.com:kraj/uClibc.git (push)
➜  uClibc git:(master) git describe --tags
v0.9.33-984-gca1c74d67
➜  uClibc git:(master) rg -t h -i "define IPV6_MULTICAST_HOPS"
libc/sysdeps/linux/common/bits/in.h
142:#define IPV6_MULTICAST_HOPS	18
➜  uClibc git:(master) rg -t h -i "define IP_MULTICAST_IF"
libc/sysdeps/linux/common/bits/in.h
35:#define IP_MULTICAST_IF 32	/* in_addr; set/get IP multicast i/f */
➜  uClibc git:(master) rg -t h -i "define IPV6_MULTICAST_IF"
libc/sysdeps/linux/common/bits/in.h
141:#define IPV6_MULTICAST_IF	17
➜  uClibc git:(master) rg -t h -i "define IPV6_UNICAST_HOPS"
libc/sysdeps/linux/common/bits/in.h
140:#define IPV6_UNICAST_HOPS	16

  ```
</details>
<details>
  <summary>FUTEX_WAIT</summary>

  ```
➜  uClibc git:(master) rg -t h -i "define FUTEX_WAIT\W"
libpthread/nptl/sysdeps/unix/sysv/linux/mips/lowlevellock.h
29:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
31:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/metag/lowlevellock.h
29:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/arc/lowlevellock.h
29:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/arm/lowlevellock.h
28:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
32:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
47:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/alpha/lowlevellock.h
29:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.h
46:#define FUTEX_WAIT		0

libpthread/nptl/sysdeps/unix/sysv/linux/sh/lowlevellock.h
29:#define FUTEX_WAIT		0

  ```
</details>
</details>
<details>
  <summary>FUTEX_PRIVATE_FLAG</summary>

  ```
➜  uClibc git:(master) rg -t h -i "define FUTEX_PRIVATE_FLAG\W"
libpthread/nptl/sysdeps/unix/sysv/linux/mips/lowlevellock.h
40:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
42:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
43:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/sh/lowlevellock.h
38:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/metag/lowlevellock.h
40:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/arm/lowlevellock.h
39:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/alpha/lowlevellock.h
40:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/arc/lowlevellock.h
40:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.h
57:#define FUTEX_PRIVATE_FLAG	128

libpthread/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
58:#define FUTEX_PRIVATE_FLAG	128

  ```
</details>
<details>
  <summary>FUTEX_WAKE</summary>

  ```
➜  uClibc git:(master) rg -t h -i "define FUTEX_WAKE\W"
libpthread/nptl/sysdeps/unix/sysv/linux/mips/lowlevellock.h
30:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/metag/lowlevellock.h
30:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/arm/lowlevellock.h
29:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
33:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/sparc/lowlevellock.h
32:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/alpha/lowlevellock.h
30:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/sh/lowlevellock.h
30:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/arc/lowlevellock.h
30:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.h
47:#define FUTEX_WAKE		1

libpthread/nptl/sysdeps/unix/sysv/linux/i386/lowlevellock.h
48:#define FUTEX_WAKE		1

  ```
</details>