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.go

@@ -52,6 +52,16 @@ func Remove(path string) error {
 	return nil
 }
 
+// Symlink is a stub, it is not implemented.
+func Symlink(oldname, newname string) error {
+	return ErrNotImplemented
+}
+
+// RemoveAll is a stub, it is not implemented.
+func RemoveAll(path string) error {
+	return ErrNotImplemented
+}
+
 // File represents an open file descriptor.
 type File struct {
 	handle FileHandle
@@ -187,6 +197,27 @@ func (e *PathError) Error() string {
 	return e.Op + " " + e.Path + ": " + e.Err.Error()
 }
 
+func (e *PathError) Unwrap() error {
+	return e.Err
+}
+
+// LinkError records an error during a link or symlink or rename system call and
+// the paths that caused it.
+type LinkError struct {
+	Op  string
+	Old string
+	New string
+	Err error
+}
+
+func (e *LinkError) Error() string {
+	return e.Op + " " + e.Old + " " + e.New + ": " + e.Err.Error()
+}
+
+func (e *LinkError) Unwrap() error {
+	return e.Err
+}
+
 const (
 	O_RDONLY int = syscall.O_RDONLY
 	O_WRONLY int = syscall.O_WRONLY