Avoid statting directories on startup for Info

Avoid statting directories on startup for Info

* doc/misc/efaq.texi (File-name conventions) Mention
Info-directory-list instead.

* doc/misc/info.texi (Emacs Info Variables): Mention
Info--default-directory-list.

* lisp/info.el (Info-default-directory-list): Default to nil to
avoid statting directories on Emacs startup (bug#27933).
(Info--default-directory-list): Factored out into own function.
(Info-default-dirs): Append value of Info-default-directory-list
to the value returned from Info--default-directory-list.