Auto merge of #451 - zethra:master, r=kamalmarhubi

Added tcgetpgrp and tcsetpgrp

I added the tcgetpgrp and tcsetpgrp functions.  I'm not sure if I did everything right so please tell me if I need to change anything.

Author: homu

CHANGELOG.md

@@ -16,6 +16,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#448](https://github.com/nix-rust/nix/pull/448))
 - Added `getpgid` in `::nix::unistd`
   ([#433](https://github.com/nix-rust/nix/pull/433))
+- Added `tcgetpgrp` and `tcsetpgrp` in `::nix::unistd`
+  ([#451](https://github.com/nix-rust/nix/pull/451))
 
 ### Changed
 - The minimum supported version of rustc is now 1.7.0.

src/unistd.rs

@@ -129,6 +129,28 @@ pub fn setsid() -> Result<pid_t> {
     Errno::result(unsafe { libc::setsid() })
 }
 
+
+/// Get the terminal foreground process group (see
+/// [tcgetpgrp(3)](http://man7.org/linux/man-pages/man3/tcgetpgrp.3.html)).
+///
+/// Get the group process id (GPID) of the foreground process group on the
+/// terminal associated to file descriptor (FD).
+#[inline]
+pub fn tcgetgrp(fd: c_int) -> Result<pid_t> {
+    let res = unsafe { libc::tcgetpgrp(fd) };
+    Errno::result(res)
+}
+/// Set the terminal foreground process group (see
+/// [tcgetpgrp(3)](http://man7.org/linux/man-pages/man3/tcgetpgrp.3.html)).
+///
+/// Get the group process id (PGID) to the foreground process group on the
+/// terminal associated to file descriptor (FD).
+#[inline]
+pub fn tcsetpgrp(fd: c_int, pgrp: pid_t) -> Result<()> {
+    let res = unsafe { libc::tcsetpgrp(fd, pgrp) };
+    Errno::result(res).map(drop)
+}
+
 /// Get the caller's thread ID (see
 /// [gettid(2)](http://man7.org/linux/man-pages/man2/gettid.2.html).
 ///