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/core/test/testpoll.c

@@ -69,7 +69,7 @@ test1()
 
     if (dcb->fd < 0)
     {
-        char errbuf[STRERROR_BUFLEN];
+        char errbuf[MXS_STRERROR_BUFLEN];
         ss_dfprintf(stderr, "\nError on function call: socket() returned %d: %s\n", errno, strerror_r(errno, errbuf,
                                                                                                       sizeof(errbuf)));
         return 1;