Fix a bug where command line arguments could be misprocessed if getopts is
used prior to f_dialog_init() -- e.g., in a script that sets
DIALOG_SELF_INITIALIZE to NULL, preventing f_dialog_init() from being run
automaticaly when `dialog.subr' is included. Caused by sub-shell processing
of arguments inheriting prior value of $OPTIND, used by getopts. Solved by
unsetting OPTIND prior to [re-]processing of positional arguments.