add "unknown" as possible value for access fields

interim/source_strings.yaml

@@ -59,6 +59,9 @@ en:
           private:
             description: Access allowed only with permission of the owner on an individual basis
             title: Private
+          unknown:
+            description: Access conditions are unknown or unclear
+            title: Unknown
           'yes':
             description: Access allowed by law; a right of way
             title: Allowed
@@ -88,6 +91,8 @@ en:
           permit: By Permit Only
           # access=private
           private: Private
+          # access=unknown
+          unknown: Unknown
           # access=yes
           'yes': Public
         # 'terms: permitted,private,public'