fix #H4039 - symset:default

fix #H4039 - symset:default

Options parsing didn't support "default" (shown by the 'O' command)
or "Default symbols" (menu entry for choosing a symbol set via the
'O' command.  Symbol handling is somewhat confusing, but this seems to
do the trick.  They can't be truncated, but they're case-insensitive,
and "Default" and "symbols" can be separated by dash or underscore as
well as space, or run-together with no separator.