LwIP: Add in support for client

New demo client app.

Update some of the memory sizes.
New coap_lwip_dump_memory_pools() to dump LwIP memory pool usage.

Add in logic to handle new client sessions.

New coap_io_process() to handle LwIP which can invoke a callback handler
to wait / timeout for the next input handler.
New coap_lwip_set_input_wait_handler() to define this handler.

Update coap_io_prepare_io() to handle LwIP.

Support re-use of pbuf for PDU re-transmissions.

Make sure that NSTART is correctly obeyed.

Build the client with client only libcoap code.

Build the server with server only libcoap code.

Update LwIP source to latest stable release.

Reserve memory for a response PDU.

New coap_lwip(3) man page.

Update directory structures for config files.

Update files contained in "make dist" and check lwip rebuilds
in the github workflows.

scripts/github_dist.sh

@@ -19,6 +19,14 @@ if test $err = 0 -a "x$ARCHIVE" != "x"; then
         $DIR/configure $PREFIX --enable-tests  --enable-silent-rules --enable-documentation --enable-examples --disable-dtls && \
         make EXTRA_CFLAGS=-Werror && make install EXTRA_CFLAGS=-Werror
     err=$?
+    if [ $err = 0 ] ; then
+        make -C $DIR/examples/lwip
+        err=$?
+    fi
+    if [ $err = 0 ] ; then
+        make -C $DIR/examples/contiki
+        err=$?
+    fi
 fi
 
 exit $err