(gdb-force-update): Rename from gdb-var-changed.

(gdb-force-update): Rename from gdb-var-changed.
(gdb-post-prompt): Use it.
(gdb-var-create-handler, gdb-var-evaluate-expression-handler)
(gdb-var-update-handler, gdb-var-delete)
(gdb-speedbar-expand-node, gdb-var-list-children-handler-1)
(gdb-var-update-handler-1): Don't set gdb-var-changed, just set
gdb-force-update in gdb-post-prompt.
(gdb-reset): Clear watch expressions from speedbar when quitting.