Fixed initialization of coap multi_option_t

is_static field wasn't initalized in case of non static option

The fixed valgrind error:

Trying to connect to LWM2M Server at localhost:5683
==23708== Conditional jump or move depends on uninitialised value(s)
==23708==    at 0x8052355: free_multi_option (er-coap-13.c:317)
==23708==    by 0x80525C8: coap_free_header (er-coap-13.c:421)
==23708==    by 0x8052C11: coap_serialize_message (er-coap-13.c:487)
==23708==    by 0x8050C91: transaction_send (transaction.c:260)
==23708==    by 0x8051156: lwm2m_register (registration.c:138)
==23708==    by 0x804ACFE: main (lwm2mclient.c:356)
==23708==
==23708== Conditional jump or move depends on uninitialised value(s)
==23708==    at 0x8052355: free_multi_option (er-coap-13.c:317)
==23708==    by 0x80525D9: coap_free_header (er-coap-13.c:422)
==23708==    by 0x8052C11: coap_serialize_message (er-coap-13.c:487)
==23708==    by 0x8050C91: transaction_send (transaction.c:260)
==23708==    by 0x8051156: lwm2m_register (registration.c:138)
==23708==    by 0x804ACFE: main (lwm2mclient.c:356)
Use CoAP transaction for deregistration

Client deregistration is now performed using a CoAP transaction.
It was previously done without, so no retry were performed, even
if the DELETE were a CON message.
Support for registration uupdate

Registration update can be used for keeping NAT route open
For now the registration update nothing in the registration
parameters, it just send a PUT on the registration path.