refactor recvmmsg lifetimes

[pacak]

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What does this PR do

Currently recvmmsg uses a single lifetime to represent several different things:

• preallocated MultiHdr
• separately allocated IoSliceMut
• iterator used to give access to all the IoSliceMut

While correct this makes it impossible to reuse preallocated headers in a loop - modified test fails to compile without this change, but compiles and runs as expected after.

As for the actual reason - I suspect this is a bug in the borrow checker, at least this RUSTFLAGS=-Zpolonius cargo +nightly check find no problems with modified test without requiring changes to recvmmsg.

Hmm... And there's an unrelated formatting only commit - current formatting is wrong, but ignored by CI.

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API