layer23: Make the logfile configurable with cell_log

layer23: Make the logfile configurable with cell_log

Be able to add extra (short) options from the 'applet' to
the main application. Use this to print the help mentioning
app specific options, pass the getopt string and handle the
command line parsing for it.

Change cell_log to keep the logname in the app_cell_log.c
and then access it from the cell_log.c implementation.