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/env.go

@@ -12,3 +12,7 @@ func Getenv(key string) string {
 func LookupEnv(key string) (string, bool) {
 	return syscall.Getenv(key)
 }
+
+func Environ() []string {
+	return syscall.Environ()
+}