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/protocol/maxscaled/maxscaled.c

@@ -162,7 +162,7 @@ static bool authenticate_socket(MAXSCALED *protocol, DCB *dcb)
     }
     else
     {
-        char errbuf[STRERROR_BUFLEN];
+        char errbuf[MXS_STRERROR_BUFLEN];
 
         MXS_ERROR("Could not get socket family of client connection: %s",
                   strerror_r(errno, errbuf, sizeof(errbuf)));