Convert more parts of Gnus from `gnus-define-keys' to `define-keymap'

Convert more parts of Gnus from `gnus-define-keys' to `define-keymap'

* lisp/gnus/spam.el (gnus-summary-mode-map):
* lisp/gnus/gnus-topic.el (gnus-topic-mode-map):
* lisp/gnus/gnus-sum.el (gnus-summary-mode-map):
* lisp/gnus/gnus-srvr.el (gnus-server-mode-map):
(gnus-server-mode-map):
(gnus-browse-mode-map):
* lisp/gnus/gnus-score.el (gnus-summary-score-map):
* lisp/gnus/gnus-draft.el (gnus-draft-mode-map):
* lisp/gnus/gnus-agent.el (gnus-agent-group-mode-map):
(gnus-agent-summary-mode-map):
(gnus-agent-server-mode-map):
(gnus-category-mode-map): Convert from `gnus-define-keys' to
`define-keymap'.

* lisp/gnus/gnus.el: Define more commands.