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/log_manager.h

@@ -21,13 +21,6 @@
 
 MXS_BEGIN_DECLS
 
-/*
- * We need a common.h file that is included by every component.
- */
-#if !defined(STRERROR_BUFLEN)
-#define STRERROR_BUFLEN 512
-#endif
-
 /**
  * If MXS_MODULE_NAME is defined before log_manager.h is included, then all
  * logged messages will be prefixed with that string enclosed in square brackets.