os: stub out support for some more features

This is necessary for the following:

  - to make sure os/exec can be imported
  - to make sure internal/testenv can be imported

The internal/testenv package (which imports os/exec) is used by a lot of
tests. By adding support for it, more tests can be run.

This commit adds a bunch of new packages that now pass all tests.

src/os/file_anyos.go

@@ -21,6 +21,8 @@ var (
 	Stderr = &File{unixFileHandle(syscall.Stderr), "/dev/stderr"}
 )
 
+const DevNull = "/dev/null"
+
 // isOS indicates whether we're running on a real operating system with
 // filesystem support.
 const isOS = true