Auto merge of #34700 - inejge:ai-hints, r=alexcrichton

Use hints with getaddrinfo() in std::net::lookup_host()

As noted in #24250, `std::net::lookup_host()` repeats each IPv[46] address in the result set. The number of repetitions is OS-dependent; e.g., Linux and FreeBSD give three copies, OpenBSD gives two. Filtering the duplicates can be done by the user if `lookup_host()` is used explicitly, but not with functions like `TcpStream::connect()`. What happens with the latter is that any unsuccessful connection attempt will be repeated as many times as there are duplicates of the address.

The program:

```rust
use std::net::TcpStream;

fn main() {
    let _stream = TcpStream::connect("localhost:4444").unwrap();
}
```

results in the following capture:

[capture-before.txt](https://github.com/rust-lang/rust/files/352004/capture-before.txt)

assuming that "localhost" resolves both to ::1 and 127.0.0.1, and that the listening program opens just an IPv4 socket (e.g., `nc -l 127.0.0.1 4444`.) The reason for this behavior is explained in [this comment](https://github.com/rust-lang/rust/issues/24250#issuecomment-92240152): `getaddrinfo()` is not constrained.

Various OSS projects (I checked out Postfix, OpenLDAP, Apache HTTPD and BIND) which use `getaddrinfo()` generally constrain the result set by using a non-NULL `hints` parameter and setting at least `ai_socktype` to `SOCK_STREAM`. `SOCK_DGRAM` would also work. Other parameters are unnecessary for pure name resolution.

The patch in this PR initializes a `hints` struct and passes it to `getaddrinfo()`, which eliminates the duplicates. The same test program as above with this change produces:

[capture-after.txt](https://github.com/rust-lang/rust/files/352042/capture-after.txt)

All `libstd` tests pass with this patch.

src/libstd/sys/common/net.rs

@@ -601,3 +601,22 @@ impl fmt::Debug for UdpSocket {
             .finish()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use prelude::v1::*;
+
+    use super::*;
+    use collections::HashMap;
+
+    #[test]
+    fn no_lookup_host_duplicates() {
+        let mut addrs = HashMap::new();
+        let lh = match lookup_host("localhost") {
+            Ok(lh) => lh,
+            Err(e) => panic!("couldn't resolve `localhost': {}", e)
+        };
+        let _na = lh.map(|sa| *addrs.entry(sa).or_insert(0) += 1).count();
+        assert!(addrs.values().filter(|&&v| v > 1).count() == 0);
+    }
+}