Rename file-subdir-of-p to file-in-directory-p.

Rename file-subdir-of-p to file-in-directory-p.

* lisp/files.el (file-in-directory-p): Rename from file-subdir-of-p.
Handle root directory properly.
(copy-directory): Caller changed.

* lisp/dired-aux.el (dired-copy-file-recursive, dired-create-files):
* lisp/net/tramp.el (tramp-file-name-for-operation): Callers changed.

* doc/lispref/files.texi (Kinds of Files): file-subdir-of-p renamed to
file-in-directory-p.