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/syscall/syscall_nonhosted.go

@@ -67,6 +67,13 @@ func Getenv(key string) (value string, found bool) {
 	return "", false
 }
 
+func Environ() []string {
+	env := runtime_envs()
+	envCopy := make([]string, len(env))
+	copy(envCopy, env)
+	return envCopy
+}
+
 func Open(path string, mode int, perm uint32) (fd int, err error) {
 	return 0, ENOSYS
 }