add setfsuid and setfsgid implementation for filesystem checks

src/unistd.rs

@@ -1398,6 +1398,26 @@ pub fn setgid(gid: Gid) -> Result<()> {
     Errno::result(res).map(drop)
 }
 
+/// Set the user identity used for filesystem checks
+///
+/// See also [setfsuid(2)](http://man7.org/linux/man-pages/man2/setfsuid.2.html)
+#[inline]
+pub fn setfsuid(uid: Uid) -> Result<()> {
+    let res = unsafe { libc::setfsuid(uid.into()) };
+
+    Errno::result(res).map(drop)
+}
+
+/// Set the group identity used for filesystem checks
+///
+/// See also [setfsgid(2)](http://man7.org/linux/man-pages/man2/setfsgid.2.html)
+#[inline]
+pub fn setfsgid(gid: Gid) -> Result<()> {
+    let res = unsafe { libc::setfsgid(gid.into()) };
+
+    Errno::result(res).map(drop)
+}
+
 /// Get the list of supplementary group IDs of the calling process.
 ///
 /// [Further reading](http://pubs.opengroup.org/onlinepubs/009695399/functions/getgroups.html)