Auto merge of #508 - APTy:ipproto-icmp, r=alexcrichton

Add IPPROTO_ICMP and IPPROTO_ICMPV6 system constants

This adds the protocol value for `ICMP` in both IPv4 and IPv6 variants.

ICMP protocol constant for IPv4 defined as `1` per [RFC 792](https://tools.ietf.org/html/rfc792)
ICMP protocol constant for IPv6 defined as `58` per [RFC 2463](https://tools.ietf.org/html/rfc2463#section-1)

#### Reference
[bsd/apple: netinet/in.h](https://opensource.apple.com/source/xnu/xnu-1699.24.8/bsd/netinet/in.h)
[bsd/freebsdlike/freebsd: netinet/in.h](https://github.com/freebsd/freebsd/blob/master/sys/netinet/in.h#L42)
[bsd/freebsdlike/dragonfly: netinet/in.h](https://github.com/DragonFlyBSD/DragonFlyBSD/blob/1f249c981c4e89e7cde1836a75b61cac36dc7ac5/sys/netinet/in.h#L64)
[bsd/netbsdlike: netinet/in.h](https://github.com/IIJ-NetBSD/netbsd-src/blob/af5d253140491f2d1816c59ecb8a4d8a8d927688/sys/netinet/in.h#L77)
[unix/haiku: netinet/in.h](https://github.com/haiku/haiku/blob/b65adbdfbc322bb7d86d74049389c688e9962f15/headers/posix/netinet/in.h#L54)
[unix/notbsd/linux: in.h](https://github.com/torvalds/linux/blob/8fa3b6f9392bf6d90cb7b908e07bd90166639f0a/include/uapi/linux/in.h#L31) - [unix/notbsd/linux: in6.h](https://github.com/torvalds/linux/blob/8fa3b6f9392bf6d90cb7b908e07bd90166639f0a/include/uapi/linux/in6.h#L134)