Code and doc fixes for file-subdir-of-p and files-equal-p.

Code and doc fixes for file-subdir-of-p and files-equal-p.

* lisp/files.el (files-equal-p): Doc fix.
(file-subdir-of-p): Doc fix.  Convert loop macro to plain Lisp,
and quit the loop once a mismatch is found.

* doc/lispref/files.texi (Kinds of Files): Improve documentation of
files-equal-p and file-subdir-of-p.