(dired-read-shell-command-default): Move it to

(dired-read-shell-command-default): Move it to
gnus/mailcap.el and change its name to more general name
mailcap-file-default-commands.
(minibuffer-default-add-dired-shell-commands): New function.
Use declare-function for mailcap-file-default-commands from "mailcap".
(dired-read-shell-command): Set local minibuffer-default-add-function
to minibuffer-default-add-dired-shell-commands in minibuffer-with-setup-hook
before calling read-shell-command.  Remove dired-read-shell-command-default
from the default value arg because default values are not set in
minibuffer-default-add-dired-shell-commands.  Doc fix.