sender: miscellaneous cleanups

sender: miscellaneous cleanups

This collects various minor syntactical changes into a single patch:
 * replace indirection 'ut->sc->xxx' by 'sc->xxx' where possible;
 * more elegant way of duplicating sc->name string;
 * udp_delete_target() now takes sender_client as argument;
 * added IPv4-address:port example for udp_help();
 * revised the definition of E_ADDRESS_LOOKUP (it means what it says)
   and replaced two inappropriate uses by assert(): in each case this
   error value was used to point to an unsupported adress family. But
   since the only supported network protocol is IP, if the case happens,
   it means a bug and should be marked as such.
--
 error.h       |    2 -
 para_audioc   |binary
 error.h       |    2 -
 send_common.c |    2 -
 udp_recv.c    |    4 ---
 udp_send.c    |   64 +++++++++++++++++++++++++++++-----------------------------
 4 files changed, 35 insertions(+), 37 deletions(-)