examples/client.c: Make use of new optlist functions
examples/client.c: Make use of new optlist functions

Remove coap_list_t structure and replace with coap_optlist_t to maintain
backward code compatability.
Make use of the new coap_optlist_* functions
Use the new coap_optlist_t structure
Remove unneeded header includes
warn() -> coap_log(LOG_WARNING, )
debug() -> coap_log(LOG_DEBUG, )

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap