add setfsgid/setfsuid on Linux

Author: Mic92

src/unix/notbsd/mod.rs

@@ -970,6 +970,8 @@ extern {
                   options: ::c_int) -> ::c_int;
     pub fn setreuid(ruid: ::uid_t, euid: ::uid_t) -> ::c_int;
     pub fn setregid(rgid: ::gid_t, egid: ::gid_t) -> ::c_int;
+    pub fn setfsgid(gid: ::gid_t) -> ::c_int;
+    pub fn setfsuid(uid: ::uid_t) -> ::c_int;
     pub fn getresuid(ruid: *mut ::uid_t, euid: *mut ::uid_t,
                      suid: *mut ::uid_t) -> ::c_int;
     pub fn getresgid(rgid: *mut ::gid_t, egid: *mut ::gid_t,