theo buehler points out that posix specifies a prompt defined using -p

theo buehler points out that posix specifies a prompt defined using -p
be reinstated when toggled off then on again; this seems to make sense,
but posix is not explicit.

we know that gnu ed and freebsd do reinstate the user defined prompt, but
we do not. no one has yet stepped up to change this, so i'm adding a note
to this effect until such a time as we do/

i have not tweaked the text for -p or "P" to allow for an easy back out
if behaviour does change;