message: Fix signature of unsetenv
diff: @@ -1023,7 +1023,7 @@ pub extern "C" fn unlockpt(fildes: c_int) -> c_int {
  }
  #[no_mangle]
  -pub unsafe extern "C" fn unsetenv(key: *mut c_char) -> c_int {
  +pub unsafe extern "C" fn unsetenv(key: *const c_char) -> c_int {
  if let Some((i, _)) = find_env(key) {
  // No need to worry about updating the pointer, this does not
  // reallocate in any way. And the final null is already shifted back.
  