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/testadminusers.c

@@ -37,7 +37,7 @@
 #include <maxscale/gwdirs.h>
 #include <maxscale/adminusers.h>
 #include <maxscale/alloc.h>
-
+#include <maxscale/skygw_utils.h>
 
 /**
  * test1    default user