UDP: Enable sending and receiving of per-packet options

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Caveat: the following options are enabled unconditionally:

• IP_RECVTTL/IPV6_RECVHOPLIMIT
• IP_RECVTOS/IPV6_RECVTCLASS
• IP_RECVPKTINFO/IPV6_RECVPKTINFO

The primary goal of these changes is to kickstart WASI's UDP capabilities and enable workloads that we couldn't run before. I doubt these 32-ish additional bytes are going to cause any performance issues for the average program, but if they ever do we can always add a "diet" version of receive without these fields.

Fixes #38

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To be reevaluated after preview2