Rebind `org-agenda-priority' to `C-c ," in agenda mode (`C-u C-c ,' now shows priority)

Rebind `org-agenda-priority' to `C-c ," in agenda mode (`C-u C-c ,' now shows priority)

* org.el (org-priority): Use a new argument to show priority
instead of setting it.
(org-show-priority): New function to show priority both in
normal Org buffers and in Org Agenda buffers.
(org-speed-commands-default): Use "," as a speed command for
setting priority.

* org-agenda.el (org-agenda-mode-map): Bind
`org-agenda-priority' to `C-c ,' as it was before.
(org-agenda-show-priority): Delete.
(org-agenda-priority): Use a new argument to show priority
instead of setting it.

Thanks to Robert Horn for triggering this change.