cgen: make os less special, fix an -autofree leak on just import os

* Improve documentation of v.util.Surrounder

* Remove `os` from the list of "no auto free" `builtin` mods

* Fix -autofree freeing of `const x = []string{}`.

* Add a valgrind regression test.

* Implement os.getenv_opt in vlib/os/environment.js.v too.

vlib/builtin/array.v

@@ -554,7 +554,7 @@ pub fn (mut a []string) free() {
 	for s in a {
 		unsafe { s.free() }
 	}
-	unsafe { free(a.data) }
+	unsafe { (&array(&a)).free() }
 }
 
 // str returns a string representation of the array of strings