unix: Add openpty(3) and forkpty(3) for non-Apple platforms

The functions were added for Apple in #202. Adding them to other
platforms was pending an amendment to RFC 1291 to expand the scope of
libc to include libutil. The amendment was merged as
  rust-lang/rfcs#1529

Author: kamalmarhubi

libc-test/build.rs

@@ -129,6 +129,12 @@ fn main() {
 
     if bsdlike {
         cfg.header("sys/event.h");
+
+        if freebsd {
+            cfg.header("libutil.h");
+        } else {
+            cfg.header("util.h");
+        }
     }
 
     if linux {
@@ -138,6 +144,7 @@ fn main() {
         cfg.header("sys/xattr.h");
         cfg.header("sys/ipc.h");
         cfg.header("sys/shm.h");
+        cfg.header("pty.h");
     }
 
     if linux || android {

src/unix/bsd/freebsdlike/mod.rs

@@ -592,6 +592,19 @@ extern {
                        info: *mut siginfo_t) -> ::c_int;
 }
 
+#[link(name = "util")]
+extern "C" {
+    pub fn openpty(amaster: *mut ::c_int,
+                   aslave: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *mut termios,
+                   winp: *mut ::winsize) -> ::c_int;
+    pub fn forkpty(amaster: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *mut termios,
+                   winp: *mut ::winsize) -> ::pid_t;
+}
+
 cfg_if! {
     if #[cfg(target_os = "freebsd")] {
         mod freebsd;

src/unix/bsd/openbsdlike/mod.rs

@@ -395,6 +395,19 @@ extern {
     pub fn fdatasync(fd: ::c_int) -> ::c_int;
 }
 
+#[link(name = "util")]
+extern "C" {
+    pub fn openpty(amaster: *mut ::c_int,
+                   aslave: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *mut termios,
+                   winp: *mut ::winsize) -> ::c_int;
+    pub fn forkpty(amaster: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *mut termios,
+                   winp: *mut ::winsize) -> ::pid_t;
+}
+
 cfg_if! {
     if #[cfg(target_os = "bitrig")] {
         mod bitrig;

src/unix/notbsd/linux/mod.rs

@@ -533,6 +533,19 @@ extern {
                        info: *mut siginfo_t) -> ::c_int;
 }
 
+#[link(name = "util")]
+extern "C" {
+    pub fn openpty(amaster: *mut ::c_int,
+                   aslave: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *const termios,
+                   winp: *const ::winsize) -> ::c_int;
+    pub fn forkpty(amaster: *mut ::c_int,
+                   name: *mut ::c_char,
+                   termp: *const termios,
+                   winp: *const ::winsize) -> ::pid_t;
+}
+
 cfg_if! {
     if #[cfg(any(target_env = "musl",
                  target_os = "emscripten"))] {