Starting with version 4.2.0, libcoap's header files are in
$prefix/coap2. This change adjusts the pre-processor includes
accordingly.

* The usage message now shows the libdcaf version.
* Added missing argument description to usage message.
* Fixed options '-A' and '-a' (now consistent with c and s):
   * `-A` address  interface address to bind to
   * `-a` URI      authorization manager (AM) URI

Configure now generates libdcaf.h from libdcaf.h.in. This file
is included in the Makefile's list of DCAF headers.

In dcaf_set_key(), a maximum key size of DCAF_MAX_KEY_SIZE is
enforced, in dcaf_set_kid() a maximum kid size of DCAF_MAX_KID_SIZE
is enforced. These functions now output an error message in case
the given key or kid, respectively, are too long.

* The functions for adding and retrieving keys now use the struct
  coap_address_t to specify the remote peer.

* The special functions for setting and retrieving an AM key
  have been removed.

As CoAP libraries such as libcoap have sorted lists of CoAP options
we do not need our own option list representation anymore. The
data type dcaf_optlist_t now is mapped to coap_optlist_t.

make now can detect that the library has changed and will rebuild
the example programs.

A handler for the unkown resource to enable creation of a RESTful
API for dynamic configuration changes.

The functions dcaf_parse_ticket_request() and dcaf_set_ticket_grant()
are now in dcaf_am.[hc].

The functions dcaf_set_am_key() and dcaf_get_am_key() now can be
used to store and retrieve AM keys. Currently, only one AM key
is kept, i.e. storing a new AM key will overwrite the old AM key.

Known bugs: There is currently no cleanup at the end of the program,
i.e. the last key's memory will be kept to the end.

When the input data cannot be parsed as CBOR, the already allocated
cose_obj_t structure must be released.

The new function coap_get_response_code() returns the response code
from the CoAP base header of the provided CoAP message. This function
is for code readability as request and response codes are stored at
the same place.