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/externcmd.c

@@ -171,7 +171,7 @@ int externcmd_execute(EXTERNCMD* cmd)
 
     if (pid < 0)
     {
-        char errbuf[STRERROR_BUFLEN];
+        char errbuf[MXS_STRERROR_BUFLEN];
         MXS_ERROR("Failed to execute command '%s', fork failed: [%d] %s",
                   cmd->argv[0], errno, strerror_r(errno, errbuf, sizeof(errbuf)));
         rval = -1;