This configuration must be used with am -H

* Write rules to database
* Evaluate rules in getTicket()

* rule.hh: define KeyId as key type for groups
* db.{hh,cc}: use KeyId in addToGroup() and findGroups()

A rule description of the form

  rules:
    - device: "dev"
      resource: "/resource"
      methods: [GET]
      allow:
        - registered
        - "other group"

now is stored in the rulebase with allow-entries for "registered" and
"other group".

Retrieve rules for given audience.

TODO: Evaluate against ticket request and group.

As type deduction for the template functions failed for the
pair-specialization of std::get<1>, transform is now called with a
lambda function to retrieve the second field.

dcaf_set_ticket_grant() now invokes a callback function to retrieve
a new dcaf_aif_t structure to include with the access ticket. The
callback function is set using dcaf_set_ticket_cb().

The Database class now provides additional member functions to
store groups and rules:

addToGroup(): adds a new pair (key, group) to the internal storage.

findGroups(): retrieves all groups for a given key.

addToRules(): Adds a new pair (audience, rule) to the internal storage.

findRules(): Retrieves all rules for a given audience.

If no pretty printing of CBOR structures is enabled but DCAF_DEBUG is set,
a hex dump of the CBOR structure is output.

Libcoap now calls dcaf_get_server_psk() with the psk_identity for
every handshake. Preset keys for devices in the security domain
therefore must be searched before the psk_identity is parsed.

If DCAF_SERVER_VALIDITY_OPTION is set to 1, no time information is
included in the SAM information message for later validty checks. This
change fixes the length information in the surrounding CBOR structure.

dcaf_set_ticket_grant() now stores the newly created ticket in
the internal ticket store and returns the pointer to this ticket
or NULL.

The new function dcaf_check_ticket() returns true if the specified
data object is present in the DCAF ticket store.

After coap_pdu_parse() has been moved back into the public API
this test case can be used again.

Libcoap version 4.3 hides PDU implementation details from
applications. This change uses the new getter functions where
provided. An open issue is the removal of coap_pdu_parse()
with no adequate replacement.

As some unit tests are currently broken, --disable-tests now is
the default. You can switch on unit tests by passing --enable-tests
to configure.

With option -H, no valid host certificate is required for the AM.
This option is intended only for testing.

With commit 38d66cc6, num_segments may be uninitialized if no query is
written. This change ensures that num_segments is always initialized
to 0.

When called with COAP_OPTION_URI_QUERY, set_uri_options() creates
Uri-Query options only for non-empty query strings.

Hugo Damer has pointed out that the assertion in make_ticket_request()
fails when buf is not cleared before calling coap_get_resource_uri().

Fix commit 3459435c that failed to include libdcaf.h in
libdcaf_include_HEADERS.

Received CoAP responses may contain SAM information messages in case
the sender is not authorized. Therefore, a response is checked for the
Content-Format DCAF_MEDIATYPE_DCAF_CBOR. If no Content-Format is set,
the response is treated as non-DCAF message.