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.

lisp/gnus/gnus-group.el

@@ -1129,8 +1129,8 @@ The following commands are available:
   (gnus-update-group-mark-positions)
   (when gnus-use-undo
     (gnus-undo-mode 1))
-  (when gnus-slave
-    (gnus-slave-mode)))
+  (when gnus-child
+    (gnus-child-mode)))
 
 (defun gnus-update-group-mark-positions ()
   (save-excursion
@@ -4024,9 +4024,9 @@ otherwise all levels below ARG will be scanned too."
     (gnus-run-hooks 'gnus-get-top-new-news-hook)
     (gnus-run-hooks 'gnus-get-new-news-hook)
 
-    ;; Read any slave files.
-    (unless gnus-slave
-      (gnus-master-read-slave-newsrc))
+    ;; Read any child files.
+    (unless gnus-child
+      (gnus-parent-read-child-newsrc))
 
     (gnus-get-unread-articles (gnus-group-default-level arg t)
 			      nil one-level)