resource.c: Add in support for unknown URI handler and update hashing
resource.c: Add in support for unknown URI handler and update hashing

Include coap_resource_unknown_init() function for defining the unknown
resource.

Update hashing to use the resource's uri_path instead of key where appropriate
to avoid clashes when using the old hashed key.

coap_add_resource() now deletes previous resource with matching uri_path to
prevent duplication.

Remove now obsolete functions
coap_hash_request_uri()
coap_get_resource_from_key()

New function
coap_get_resource_from_uri_path()

*API Update coap_delete_resource() now deletes by resource, not by key.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap