Add PS_SIGWINCH_PEND (mutt#3837 by flachkapp)..
Flachkapp is absolutely right in that we don't update $COLUMNS and
$LINES after screen resizes.
These values are not normally exported by the shell, but since we
want to provide a friendly environment for the child processes
we'll manage, and do manage the screen dimensions in interactive
mode anyway, let's just export $COLUMNS and $LINES: like that
users don't need to do `stty size`, which (the "size" of it)
furthermore isn't standardized by POSIX.