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_unix.go

@@ -2,4 +2,23 @@
 
 package os
 
+import "syscall"
+
 type syscallFd = int
+
+func Pipe() (r *File, w *File, err error) {
+	var p [2]int
+	err = handleSyscallError(syscall.Pipe2(p[:], syscall.O_CLOEXEC))
+	if err != nil {
+		return
+	}
+	r = &File{
+		handle: unixFileHandle(p[0]),
+		name:   "|0",
+	}
+	w = &File{
+		handle: unixFileHandle(p[1]),
+		name:   "|1",
+	}
+	return
+}