Add types for inspecting control messages in Unix socket ancillary data

[jmillikin]

Tracking issue: #76915

RFC for unix_socket_ancillary_data redesign: rust-lang/rfcs#3430

The portion of the new API dealing with control message inspection seems to be non-contentious, so this PR implements it. The types and logic are all separate from the current implementation, so they can co-exist without worries about compatibility or unexpected behavior changes.

New types:

• ControlMessage<'a> is a borrowed reference to a single control message. No inspection of the message content, just the generic metadata (cmsg_level, cmsg_type) and data slice.
• &ControlMessages is a wrapper around &[u8] that has an iter() -> ControlMessagesIter method.
• ControlMessagesIter<'a> iterates over control messages by inspecting cmsg_len

Includes docs and tests.

[rustbot]

r? @cuviper

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[jmillikin]

@rustbot label +T-libs-api -T-libs

[cuviper]

r? libs-api

[dtolnay]

Reassigning to Josh, who has opinions about the existing ancillary data API and the proposed one.

r? joshtriplett

[jmillikin]

CI is green now (having done its job of reminding me that UI tests exist...)

[jmillikin]

Note: the most recent iteration of the RFC doesn't require ControlMessagesBuf, so I've removed it from this PR.

[joshtriplett]

r? libs-api