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/debugcli/debugcli.c

@@ -293,7 +293,7 @@ execute(ROUTER *instance, void *router_session, GWBUF *queue)
     {
         const char* data = GWBUF_DATA(queue);
         int len = GWBUF_LENGTH(queue);
-        int n = MIN(len, CMDBUFLEN - cmdlen - 1);
+        int n = MXS_MIN(len, CMDBUFLEN - cmdlen - 1);
 
         if (n != len)
         {