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.

include/maxscale/externcmd.h

@@ -18,7 +18,6 @@
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>
-#include <maxscale/skygw_utils.h>
 #include <maxscale/log_manager.h>
 #include <maxscale/pcre2.h>