Merge #1744

1744: reimplement sendmmsg/recvmmsg with better API r=rtzoeller a=pacak

Motivation is explained in #1602, new version allows to receive data without performing allocations inside the receive loop and to use received data without extra copying.

This pull request contains a breaking change to API `recvmmsg` (obviously) and also affects `recvmsg` - new version does not set length of control message buffer if one is passed. Later change can be avoided with a bit more copy-paste.

Fixes #1602

Co-authored-by: Michael Baikov <manpacket@gmail.com>

Author: bors[bot]

CHANGELOG.md

@@ -85,6 +85,9 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 ### Changed
 
+- Reimplemented sendmmsg/recvmmsg to avoid allocations and with better API
+  (#[1744](https://github.com/nix-rust/nix/pull/1744))
+
 - Rewrote the aio module.  The new module:
   * Does more type checking at compile time rather than runtime.
   * Gives the caller control over whether and when to `Box` an aio operation.