Add setresuid and setresgid

These were both recently added to libc, add wrappers.

Signed-off-by: Dylan Reid <dgreid@chromium.org>

src/unistd.rs

@@ -568,6 +568,7 @@ pub fn mkstemp<P: ?Sized + NixPath>(template: &P) -> Result<(RawFd, PathBuf)> {
 
 #[cfg(any(target_os = "linux", target_os = "android"))]
 mod linux {
+    use libc::{self, uid_t, gid_t};
     use sys::syscall::{syscall, SYSPIVOTROOT};
     use {Errno, Result, NixPath};
 
@@ -587,6 +588,20 @@ mod linux {
         Errno::result(res).map(drop)
     }
 
+    #[inline]
+    pub fn setresuid(ruid: uid_t, euid: uid_t, suid: uid_t) -> Result<()> {
+        let res = unsafe { libc::setresuid(ruid, euid, suid) };
+
+        Errno::result(res).map(drop)
+    }
+
+    #[inline]
+    pub fn setresgid(rgid: gid_t, egid: gid_t, sgid: gid_t) -> Result<()> {
+        let res = unsafe { libc::setresgid(rgid, egid, sgid) };
+
+        Errno::result(res).map(drop)
+    }
+
     #[inline]
     #[cfg(feature = "execvpe")]
     pub fn execvpe(filename: &CString, args: &[CString], env: &[CString]) -> Result<()> {