MXS-1405: Log subprocess output immediately

When the subprocess outputs a line, the message should be logged
immediately. This allows automated timestamps for the output of the
executed subprocess.

server/core/maxscale/externcmd.h

@@ -58,11 +58,10 @@ void externcmd_free(EXTERNCMD* cmd);
  * The output of the command must be freed by the caller by calling MXS_FREE.
  *
  * @param cmd  Command to execute
- * @param dest Pointer where to store the output of the command
  *
  * @return The return value of the executed command or -1 on error
  */
-int externcmd_execute(EXTERNCMD* cmd, char** dest);
+int externcmd_execute(EXTERNCMD* cmd);
 
 /**
  * Substitute all occurrences of @c match with @c replace in the arguments for @c cmd