bug#27583: 24.5; `info.el': some test for (eq major-mode 'Info-mode)

[Drew Adams <drew.adams@oracle.com>]

Most of the places where `major-mode' is tested use a test like this:
(derived-mode-p 'Info-mode).  But there are still a few places where
this is done: (eq major-mode 'Info-mode).

I think (but am not positive) that each of the latter tests should be
changed to use `derived-mode-p', to let users and libraries define and
use modes that are derived from `Info-mode'.

It looks like the places that still use (eq major-mode 'Info-mode)
constitute code that doesn't get much love.  The occurrences are in
these 3 things:

 Info-hide-note-references (in the option's :set function)
 info-display-manual
 info--manual-names (used only by info-display-manual)



In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'