Remove dependency on skygw_utils.h

- STRERROR_BUFLEN moved to cdefs.h and renamed to MXS_STRERROR_BUFLEN.
  Better would be to provide a 'const char* mxs_strerror(int errno)'
  that would have a thread specific buffer for the error message.
- MIN and MAX also moved to defs.h as MXS_MIN and MXS_MAX.
- Now only mlist.h of the headers depend upon skygw_utils.h.

server/modules/routing/schemarouter/schemarouter.c

@@ -2004,7 +2004,7 @@ static int routeQuery(ROUTER* instance,
     {
         uint8_t* packet = GWBUF_DATA(querybuf);
         unsigned char ptype = packet[4];
-        size_t len = MIN(GWBUF_LENGTH(querybuf),
+        size_t len = MXS_MIN(GWBUF_LENGTH(querybuf),
                          MYSQL_GET_PACKET_LEN((unsigned char *)querybuf->start) - 1);
         char* data = (char*)&packet[5];
         char* contentstr = strndup(data, len);