Rework changing pane titles.

Rework changing pane titles.

If PROMPT_COMMAND is set to something which tries to change a window's
title, tmux would happily override the user-supplied pane name even
though set-titles is set to off.

Setting allow-rename to off prevents titles from being set to annoying
stuff like "[root@localhost ~]#" when becoming root on a Red Hat
machine.

Previously "automatic-rename = off" was commented out because
allow-rename overrode it.  Having an automatic name based on the current
process is acceptable in case where no explicit title has been set, so
we now set it on.