Add binding for res_init.

Some notes about the particularities of the changes:

res_init has been deprecated in favor of res_ninit, and many (but not
all) targets have therefore renamed the link name to __res_init. For
example, this happened in glibc in version 2.2:
https://bugzilla.redhat.com/show_bug.cgi?id=43822#c6
In these systems, res_init is #defined to __res_init in resolv.h, which
lets existing C programs continue to be compiled.

Unfortunately, this define doesn't automatically apply to our Rust code.
We therefore need to manually map the link name of res_init as
appropriate for each target:

macOS and iOS use res_9_init: https://github.com/practicalswift/osx/blob/3908694d6328baa293f0d7fc337348c01d13ed8f/src/libresolv/resolv.h#L316
Solaris uses res_init: https://java.net/projects/solaris/sources/on-src/content/usr/src/head/resolv.h
OpenBSD uses __res_init: https://github.com/openbsd/src/blob/f3b3b7c7ca9a921db3a5650eed40f2b2e4d731d8/include/resolv.h#L268
FreeBSD uses __res_init: https://github.com/freebsd/freebsd/blob/6911f4a88c9832e5985b788f5e84010424f9e020/include/resolv.h#L290
NetBSD uses __res_init: http://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/include/resolv.h
glibc uses __res_init: https://github.com/lattera/glibc/blob/a2f34833b1042d5d8eeb263b4cf4caaea138c4ad/resolv/resolv.h#L259
eglibc uses __res_init: https://github.com/Xilinx/eglibc/blob/7f0bcce417c47aefad06ddfec7cd4ced3a4e10ff/resolv/resolv.h#L259
musl uses res_init: https://github.com/runtimejs/musl-libc/blob/0a11d7cb13e243782da36e2e5747b8b151933cca/include/resolv.h#L128
Android uses res_init: https://github.com/android/platform_bionic/blob/306ea559528255e19a5bcd68cc2a9b1afc2cfb27/libc/include/resolv.h#L57

One caveat here is that NetBSD doesn't seem to use the symbol name
__res_init yet, despite the redefine being present in resolv.h. At least
Travis fails for the netbsd target if __res_init is used. This may
change in the future.

iOS and macOS both unfortunately require linking with libresolv, despite
the symbols technically being available without libresolv:
resolv: http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html
Android and OpenBSD fall in the same category.

Author: jonhoo

libc-test/build.rs

@@ -77,6 +77,7 @@ fn main() {
         cfg.header("netinet/in.h");
         cfg.header("netinet/ip.h");
         cfg.header("netinet/tcp.h");
+        cfg.header("resolv.h");
         cfg.header("pthread.h");
         cfg.header("dlfcn.h");
         cfg.header("signal.h");

src/unix/mod.rs

@@ -206,9 +206,22 @@ pub const PRIO_MAX: ::c_int = 20;
 cfg_if! {
     if #[cfg(dox)] {
         // on dox builds don't pull in anything
-    } else if #[cfg(all(not(stdbuild), feature = "use_std"))] {
+    } else if #[cfg(all(not(stdbuild),
+                        feature = "use_std",
+                        not(any(target_os = "macos",
+                                target_os = "ios",
+                                target_os = "android",
+                                target_os = "openbsd",
+                                target_os = "bitrig")
+                        )))] {
         // cargo build, don't pull in anything extra as the libstd  dep
         // already pulls in all libs.
+    } else if #[cfg(all(not(stdbuild), feature = "use_std"))] {
+        // except on macOS and iOS, where we must link with lib resolv
+        // for res_init, despite libsystem_info including it:
+        // http://blog.achernya.com/2013/03/os-x-has-silly-libsystem.html
+        #[link(name = "resolv")]
+        extern {}
     } else if #[cfg(any(all(target_env = "musl", not(target_arch = "mips"))))] {
         #[link(name = "c", kind = "static", cfg(target_feature = "crt-static"))]
         #[link(name = "c", cfg(not(target_feature = "crt-static")))]
@@ -229,6 +242,7 @@ cfg_if! {
                         target_os = "bitrig"))] {
         #[link(name = "c")]
         #[link(name = "m")]
+        #[link(name = "resolv")]
         extern {}
     } else if #[cfg(target_os = "haiku")] {
         #[link(name = "root")]
@@ -694,6 +708,17 @@ extern {
                        res: *mut *mut addrinfo) -> ::c_int;
     pub fn freeaddrinfo(res: *mut addrinfo);
     pub fn gai_strerror(errcode: ::c_int) -> *const ::c_char;
+    #[cfg_attr(all(unix,
+                   not(target_os = "macos"),
+                   not(target_os = "ios"),
+                   not(target_os = "netbsd"),
+                   not(target_os = "solaris"),
+                   not(target_env = "musl")
+                   ),
+               link_name = "__res_init")]
+    #[cfg_attr(any(target_os = "macos", target_os = "ios"),
+               link_name = "res_9_init")]
+    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;