Previously the filename was calculated twice: first when opening the
file, and then when displaying the message.  There is a small chance
that the time would change between those two events, causing the
displayed filename to be incorrect.  This change calculates the name
once and then uses that name in both places.

Use a pane option variable, and remove unnecessary script variables.

Set it in start_pipe_pane, unset it in stop_pipe_pane, and determine
whether logging is on by whether it is set or unset.

Since the string stored in @logging-variable can be arbitrary, let's
store the filename there, so that we can display the correct name when
we stop logging.

The name of the file that the current pane is logging to is stored in
the variable, so name it (and the helper functions) accordingly.