bug#16354: 24.3.50; `M-o' conflict in Dired - change Omit mode to `C-x M-o'

[Juri Linkov]

> dired-x.el has bound `M-o' to `dired-omit-mode', which is a toggle,
> since Time Immemorial.  The facemenu commands were originally on
> prefix key `M-g'.

dired-x.el solves a similar problem for the conflicting key M-g
by using the upper-case G:

(define-key dired-mode-map "\M-G" 'dired-goto-subdir)

so doing the same we could use upper-case M-O for dired-omit-mode.
Actually dired-x.el already uses the upper-case O for the related command:

(define-key dired-mode-map "*O" 'dired-mark-omitted)

But before taking M-O into use it should be tested to work in emacs -nw
without problems.