Auto merge of #585 - jonhoo:res_init, r=alexcrichton

Add res_init

The `res_init` function, while deprecated, is critical for making networked applications work correctly when switching between networks, or between being offline and online. By default, `getaddrinfo` and friends use a cached version of `/etc/resolv.conf`, which means that network changes are not picked up by applications after they first start up. This has bitten [Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=214538), [Pidgin](https://developer.pidgin.im/ticket/2825), [MongoDB](https://jira.mongodb.org/browse/DOCS-5700), and more in the past.

The logic behind exposing only `res_init` is that it is the only `res_*` function that is frequently called directly by user applications. The other `res_*` functions provide low-level access to domain lookups, whereas `res_init` is useful even if the application itself is not concerned with doing lookups.

Behind the scenes, `getaddrinfo` in glibc [ultimately calls](https://github.com/lattera/glibc/blob/a2f34833b1042d5d8eeb263b4cf4caaea138c4ad/resolv/nss_dns/dns-host.c#L196) `res_nsearch` with `&_res`. `res_init` operates directly on this global reference, and is thus more useful to expose for most applications than the non-deprecated `res_ninit` function (which operators on an arbitrary `res_state`).

As far as I can tell, `res_init` is available in [FreeBSD](https://www.freebsd.org/cgi/man.cgi?query=res_init&manpath=SuSE+Linux/i386+11.3), [NetBSD](http://netbsd.gw.com/cgi-bin/man-cgi?res_init+3+NetBSD-6.1), [OpenBSD](http://man.openbsd.org/resolver.3), [Solaris](http://www.polarhome.com/service/man/?qf=res_init&tf=2&of=Solaris&sf=), [Linux](https://linux.die.net/man/3/res_init), and [macOS](https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man3/res_init.3.html).

src/unix/mod.rs

@@ -694,6 +694,7 @@ extern {
                        res: *mut *mut addrinfo) -> ::c_int;
     pub fn freeaddrinfo(res: *mut addrinfo);
     pub fn gai_strerror(errcode: ::c_int) -> *const ::c_char;
+    pub fn res_init() -> ::c_int;
 
     #[cfg_attr(target_os = "netbsd", link_name = "__gmtime_r50")]
     pub fn gmtime_r(time_p: *const time_t, result: *mut tm) -> *mut tm;