Convert receivers to lopsub.

Convert receivers to lopsub.

This converts the four receivers (afh, http, dccp, udp) to use the
lopsub library instead of gengetopt.

The command line options of the receivers are implemented as
subcommands of the new recv_cmd lopsub suite. Hence the four gengetopt
command line parsers and ->ggo_help of struct receiver can be removed.

This change allows to get rid of the receiver array which was defined
through the DEFINE_RECEIVER_ARRAY macro.  We now store each receiver
structure in the user_data pointer provided by lopsub.

Since this structure is initialized at compile time (and constant since
->ggo_help is gone), ->init() of struct receiver is no longer needed to
initialize the various function pointers. The function is now optional
and does not take an argument any more. At the moment, only the afh
receiver needs ->init() to initialize all supported audio format handlers.

t0005 needs slight adjustment since the section headers of the help text
have changed a bit.