std: Unconditionally close all file descriptors

The logic for only closing file descriptors >= 3 was inherited from quite some
time ago and ends up meaning that some internal APIs are less consistent than
they should be. By unconditionally closing everything entering a `FileDesc` we
ensure that we're consistent in our behavior as well as robustly handling the
stdio case.

src/test/run-pass/fds-are-cloexec.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // ignore-windows
+// ignore-android
 
 #![feature(libc)]