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.
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