RFC8768: Add in support and update Proxy-Uri Support

Add in support for RFC8768 Hop Limit and add in code to better support
the Proxy-Uri CoAP Option.

README.md:

Add in RFC8768 is supported by libcoap.

examples/client.c:

Add in the -H hoplimit option.
Make sure that the Proxy-Uri: option is added (if set) when clearing down an
observation (fixes obgm#112).
Check that Proxy-Uri: option has a maximum data length of 1034 bytes.
Always add in Hop-Limit: CoAP option if Proxy-Uri: is in use.
Handle user defined Proxy-Scheme: option correctly.
Update -P option to include scheme to be used when connecting to the proxy.

examples/coap-server.c:

Add in hnd_proxy_uri() handler function. This has support for a direct or ongoing
proxy connection for the coap* protocols. The ongoing connection is set up as a
different CoAP session.

Internal functions taken from client.c are used where appropriate.

Proxy support code has a SERVER_CAN_PROXY wrapper, so footprint can easily be
reduced if required.

New -P and -u options added to support ongoing connections.

include/coap2/net.h:

Add in tracking the proxy uri handler into coap_context_t.

include/coap2/pdu.h:

Add in the default Hop-Limit: option COAP_OPTION_HOP_LIMIT.
Re-organize the COAP_OPTION_ descriptions to include the C, U, N and R flags.

include/coap2/resource.h:

Track whether a resource definition is for Proxy-Uri:.
Define the coap_resource_proxy_uri_init() function for handling the Proxy-Uri:
option or Proxy-Scheme: option in an incoming request.

include/coap2/uri.h:

Add in COAP_URI_SCHEME_HTTP and COAP_URI_SCHEME_HTTPS as valid Proxy-Uri:
schemes.
Define the coap_split_proxy_uri() function (similar to coap_split_uri() but
supports the proxy extras.  Both now use a common sub function internally.

libcoap-2.map:
libcoap-2.sym:

Expose the new coap_resource_proxy_uri_init() and coap_split_proxy_uri()
functions.

man/Makefile.am:

Handle coap_resource_get_userdata(3) being over the 10 definition limit in
man/coap_resource.txt.in.

man/coap-client.txt.in:

Document the new -H hoplimit option.
Update documentation for the -P option.

man/coap-server.txt.in:

Document the new -P proxy option.
Document the new -u user option.

man/coap.txt.in:

Include the new RFC8768.

man/coap_resource.txt.in:

Include documentation for coap_resource_proxy_uri_init().
Update documentation for the CoAP options.

src/coap_debug.c:

Add in support for printing out COAP_OPTION_HOP_LIMIT.
Output the Proxy-Scheme: Option as text.

src/net.c:

Make sure that COAP_OPTION_HOP_LIMIT is not returned in an error response.
Sanity check and handle COAP_OPTION_PROXY_URI and COAP_OPTION_PROXY_SCHEME
in handle_request().

src/pdu.c:

Report on CoAP options that are repeated, but not defined as repeatable.
Define new (5.08) Hop Limit Reached error response.

src/resource.c:

Add in coap_resource_proxy_uri_init() function.
Handle the addition of a proxy uri handler in coap_add_resource().

src/uri.c:

Rename coap_split_uri() to coap_split_uri_sub() and add in extra proxy
handling support based on input parameters.
Create new coap_split_uri() and coap_split_proxy_uri() which call
coap_split_uri_sub() appropriately.

README.md

@@ -46,6 +46,9 @@ The following RFCs are supported
 * RFC8132: PATCH and FETCH Methods for the Constrained Application Protocol (CoAP)
 
 * RFC8323: CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
+  [No WebSockets support]
+
+* RFC8768: Constrained Application Protocol (CoAP) Hop-Limit Option
 
 There is (D)TLS support for the following libraries