refactor recvmmsg lifetimes

[pacak]

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:

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

[pacak]

Clipy/rustdoc looks very unhappy. Anyway, we've been using this in production since I implemented it. Renamed lifetimes a bit and replaced 'data with 'iter- to cover all the references it needs to hold. What needs to be done to get this merged?