Convert writers to lopsub.

Convert writers to lopsub.

Similar to the previous commits for receivers and filters, this
commit replaces the five gengetopt parsers for the alsa, ao, file,
oss, osx writers by a lopsub suite and links para_write with -llopsub.

This allows to get rid of the WRITER_ENUM and the writers array as a
reference to each writer structure are stored in the lopsub user_data
pointer. Moreover, ->init(), ->parse_config(), ->free_config()
and ->ggo_help() of struct writer are not needed any more and can
be removed.

The patch also removes write_common.h and moves the few prototypes
write.h.

Now that receivers, filters and writers have all been converted, we
may also stop to include ggo.h from audiod_command.c and play.c.

As for the receivers and filters, t0005 needs slight adjustments due
to the new section header in the man page.