libstd/net: Add peek APIs to UdpSocket and TcpStream

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.

src/libstd/lib.rs

@@ -275,6 +275,7 @@
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
 #![feature(panic_unwind)]
+#![feature(peek)]
 #![feature(placement_in_syntax)]
 #![feature(prelude_import)]
 #![feature(pub_restricted)]