New user option 'shell-command-guess-functions' (bug#18132)

New user option 'shell-command-guess-functions' (bug#18132)

* lisp/dired-aux.el (dired-minibuffer-default-add-shell-commands):
Remove function since now mailcap commands are available
by shell-command-guess-mailcap for shell-command-guess
used by dired-guess-shell-command.
(dired-read-shell-command): Don't set minibuffer-default-add-function
to dired-minibuffer-default-add-shell-commands.
(dired-guess-shell-command): Replace dired-guess-default
with shell-command-guess.
(shell-command-guess-functions): New defcustom.
(shell-command-guess, shell-command-guess-dired)
(shell-command-guess-mailcap, shell-command-guess-xdg):
New functions.

* lisp/simple.el (minibuffer-default-add-shell-commands):
Use 'shell-command-guess' instead of requiring 'mailcap'
with 'mailcap-file-default-commands'.  Remove 'interactive'.