Auto merge of #433 - kiddkai:getpgid, r=posborne

add getpgid call

Add a `getpgid` function to nix.

Argument is required, either pass in `None` or `Some(pid_t)`. It should have the same behavior
as the `libc` one.

CHANGELOG.md

@@ -14,6 +14,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#410](https://github.com/nix-rust/nix/pull/410))
 - Added `setresuid` and `setresgid` for Linux in `::nix::unistd`
   ([#448](https://github.com/nix-rust/nix/pull/448))
+- Added `getpgid` in `::nix::unistd`
+  ([#433](https://github.com/nix-rust/nix/pull/433))
 
 ### Changed
 - The minimum supported version of rustc is now 1.7.0.

src/unistd.rs

@@ -62,6 +62,11 @@ pub fn setpgid(pid: pid_t, pgid: pid_t) -> Result<()> {
     let res = unsafe { libc::setpgid(pid, pgid) };
     Errno::result(res).map(drop)
 }
+#[inline]
+pub fn getpgid(pid: Option<pid_t>) -> Result<pid_t> {
+    let res = unsafe { libc::getpgid(pid.unwrap_or(0 as pid_t)) };
+    Errno::result(res)
+}
 
 #[cfg(any(target_os = "linux", target_os = "android"))]
 #[inline]