Replace XField integer 'flags' with string 'subtype'

include/xchange.h

@@ -140,17 +140,17 @@ typedef struct XField {
   char *name;               ///< Pointer to a designated local name buffer. It may not contain a separator (see X_SEP).
   char *value;              ///< Pointer to designated local string content (or structure)...
   XType type;               ///< The underlyng data type
+  char *subtype;            ///< Descriptive subtype, such a a mime type (if any)
   int ndim;                 ///< The dimensionality of the data
   int sizes[X_MAX_DIMS];    ///< The sizes along each dimension
-  uint32_t flags;           ///< (optional) flags that specific data exchange mechanism may use, e.g. for BSON subtype.
   boolean isSerialized;     ///< Whether the fields is stored in serialized (string) format.
   struct XField *next;      ///< Pointer to the next linked element (if inside an XStructure).
 } XField;
 
 /**
  * Static initializer for the XField data structure.
   */
-#define X_FIELD_INIT        {NULL}
+#define X_FIELD_INIT        {NULL, NULL, X_UNKNOWN, NULL, 0, {0}, FALSE, NULL}
 
 /**
  * \brief SMA-X structure object, containing a linked-list of XField elements.

src/xstruct.c

@@ -813,6 +813,7 @@ void xClearField(XField *f) {
   }
 
   if(f->name != NULL) free(f->name);
+  if(f->subtype) free(f->subtype);
 
   memset(f, 0, sizeof(XField));
 }