Refactored YANG min/max validation code, created new clixon_validate_minmax.[ch]

Added new recursive minmax check for non-presence containers
This makes validation checks stricter, including check of incoming RPCs
Renamed xml_yang_check_list_unique_minmax() to xml_yang_minmax_recurse()
Fixed again: [YANG min-elements within non-presence container does not work](https://github.com/clicon/clixon/issues/355)

test/test_helloworld.sh

@@ -134,7 +134,7 @@ if [ $? -ne 0 ]; then
     err 0 $r
 fi  
 if [ "$ret" != "clixon-hello:hello     world;" ]; then
-    err "$ret" "clixon-hello:hello     world;"
+    err "clixon-hello:hello     world;" "$ret"
 fi
 
 new "netconf edit-config"