Make GCC to remote groups work when unplugged

Make GCC to remote groups work when unplugged
(postpone GCC until message is actually sent).

* gnus-draft.el (gnus-draft-send): Call `gnus-agent-restore-gcc'.

* gnus-agent.el (gnus-agent-possibly-do-gcc):
(gnus-agent-restore-gcc):
(gnus-agent-possibly-save-gcc): New functions.

* gnus-msg.el (gnus-inews-add-send-actions): Use
`gnus-agent-possibly-do-gcc' if Agentized.
(gnus-inews-add-send-actions): Add `gnus-agent-possibly-save-gcc'
to `message-header-hook'.

* gnus.el (gnus-agent-gcc-header): New variable.