client-coap.c: Fix client spin loop for NO_SYS == 0

Do not call coap_free_context() from within sigint handler.
Always call coap_io_process().
Use sys_arch_sem_wait() for timeouts while waiting for input.

include/coap3/coap_io_internal.h

@@ -218,6 +218,8 @@ void coap_packet_get_memmapped(coap_packet_t *packet,
 
 #ifdef WITH_LWIP
 void coap_io_process_timeout(void *arg);
+void coap_io_lwip_init(void);
+void coap_io_lwip_cleanup(void);
 #endif
 
 struct coap_packet_t {