Rollup merge of #38983 - APTy:udp-peek, r=aturon

Add peek APIs to std::net

Adds "peek" APIs to `std::net` sockets, including:
- `UdpSocket.peek()`
- `UdpSocket.peek_from()`
- `TcpStream.peek()`

These methods enable socket reads without side-effects. That is, repeated calls to `peek()` return identical data. This is accomplished by providing the POSIX flag `MSG_PEEK` to the underlying socket read operations.

This also moves the current implementation of `recv_from` out of the platform-independent `sys_common` and into respective `sys/windows` and `sys/unix` implementations. This allows for more platform-dependent implementations where necessary.

Fixes #38980

src/libstd/lib.rs

@@ -277,6 +277,7 @@
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
 #![feature(panic_unwind)]
+#![feature(peek)]
 #![feature(placement_in_syntax)]
 #![feature(prelude_import)]
 #![feature(pub_restricted)]