Convert para_play to lopsub.

This changes para_play to use the lopsub library rather than gengetopt
for parsing the command line and the config file.

The old approach of permuting argv[] no longer works with lopsub
because direct access to the array of non-option arguments is no
longer possible. Therefore we introduce shuffle_map[], an array of
indices which defaults to 1..n if --randomize is not given, and a
random permutation of 1..n otherwise. The new get_playlist_file()
helper returns the path to the input file determined by permuting
the given index.

The overall description for the manual page has also been improved
slightly.

Since m4/gengetopt/history_file.m4 has become unused, this file can
be removed.