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/buffer.c

@@ -121,7 +121,7 @@ gwbuf_alloc(unsigned int size)
 retblock:
     if (rval == NULL)
     {
-        char errbuf[STRERROR_BUFLEN];
+        char errbuf[MXS_STRERROR_BUFLEN];
         MXS_ERROR("Memory allocation failed due to %s.",
                   strerror_r(errno, errbuf, sizeof(errbuf)));
     }
@@ -964,7 +964,7 @@ size_t gwbuf_copy_data(GWBUF *buffer, size_t offset, size_t bytes, uint8_t* dest
 
                 if (buffer)
                 {
-                    bytes_left = MIN(GWBUF_LENGTH(buffer), bytes);
+                    bytes_left = MXS_MIN(GWBUF_LENGTH(buffer), bytes);
                     ptr = (uint8_t*) GWBUF_DATA(buffer);
                 }
             }