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_db.sh

@@ -189,7 +189,7 @@ EOF
 new "test params: -f $cfg"
 
 for format in xml json; do
-    for pretty in false true json; do
+    for pretty in false true; do
 	new "test db $format pretty=$pretty"
 	testrun xml false
     done