Add support for TTL and RECV_ERR control messages

[pmalmgren]

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This patch adds support for the following in Linux:

• Reading the IP ttl header from a control message if IP_RECVTTL is set on the socket with setsockopt
• Reading any errors from a control message if IP_RECVERR is set on the socket with setsockopt

[pmalmgren]

@asomers Correct, it adds support for two new types of cmsg. They are requested with setsockopt and require new socket options. I'll add a commit for those to this PR.

[cemeyer]

N.B., #1161 also attempts to add IP_RECVERR support. I have not looked at either PR's changes yet, and the other PR doesn't attempt to cover IP_RECVTTL.