Convert filters to lopsub.

Convert filters to lopsub.

This replaces the *_filter.m4 gengetopt files by the filter_cmd
lopsub suite, where each filter is realized as a subcommand. Due to
this change, para_filter needs to be linked with -llopsub.

The filter structure is now stored in the user_data pointer provided
by lopsub, allowing to get rid of the global filters[] array, the
FILTER_ENUM macro and the corresponding enumeration constants.

The removal of the ->goo_help member of struct filter makes this
structure constant. Hence ->init() of struct filter can also go
away. We still can tell whether a filter is supported by checking
the user_data pointer: if it is NULL, the filter is unsupported. The
new filter_supported() helper in filter_common.c is provided for
convenience.

Parsing of the filter command line options is now performed
generically, and the ->parse_config() method is renamed to ->setup(),
an optional function which is supposed to perform semantic checks
and the one-time setup of the filter, if any. It is accompanied by
->teardown() which replaces ->free_config().

The conversion of the individual filters is easy since most filters
have a simple syntax or take no arguments at all. The resample_filter,
however, needs a different way to copy the wav parameters from the
lopsub parse result to the wav parms structure. A suitable macro,
LLS_COPY_WAV_PARMS is added to check_wav.h for this purpose. The old
COPY_WAV_PARMS needs to stay until para_write, the only other user
of the macro, has been converted as well.

The section heading of the manual page has changed slightly, causing
t0005 to fail. Hence this test needs a slight adjustment.