dired-x.el misc cleanup.

dired-x.el misc cleanup.

* lisp/dired-x.el: Simplify commentary.
Don't require man when compiling.
(dired-omit-extensions, dired-local-variables-file)
(dired-x-hands-off-my-keys): Make them defcustoms.
(Man-support-local-filenames, Man-getpage-in-background): Declare.
(vm-visit-folder): Declare rather than defining.
(dired-x-help-address, dired-x-variable-list): Remove.
(dired-x-submit-report): Make it an obsolete alias.