Adjust naming convention for dependent Gnus sessions

Adjust naming convention for dependent Gnus sessions

* doc/misc/gnus.texi (Child Gnusae): Rename node from "Slave
Gnusae" and adjust wording.  Also remove at least one joke.

* lisp/gnus/gnus-start.el (gnus-no-server-1, gnus-1)
(gnus-setup-news, gnus-save-newsrc-file): Adjust parameter names.
(gnus-child-mode): Rename and add alias.
(gnus-child-save-newsrc): Rename.
(gnus-parent-read-child-newsrc): Ditto.

* lisp/gnus/gnus-group.el (gnus-group-mode)
(gnus-group-get-new-news): Ditto.

* lisp/gnus/gnus-agent.el (gnus-child-unplugged): Rename function
from gnus-slave-unplugged (and add the latter as an obsolete alias).

* lisp/gnus/gnus.el (gnus-other-frame-function): Adjust function
names in the defcustom to new naming scheme.
(gnus-other-frame-resume-function): Ditto.
(gnus): Adjust parameter names.