Convert para_filter to lopsub.

Convert para_filter to lopsub.

Changes the semantics in case --filter is not given. We used to
print an error, but it makes more sense to show the list of supported
filters in this case.

If no filters are given, we now print the list of available filters. A
new function in filter_common.c, print_filter_list(), is introduced
for this purpose.

The manual page of para_filter has been extended to include an overall
description of the command.

The only error code of filter.c, E_NO_FILTERS, becomes unused and is
removed.