Match NSLog read UID functionality in DDASLLogger

`NSLog` sets the read UID property of ASL messages to the current EUID.  This pull request changes `DDASLLogger` to match the `NSLog` functionality.

Currently, `DDASLLogger` *always* sets the read UID property to 501, which I believe is typically the UID of the first user account after a fresh install of OS X.

The asl(3) library calls are also now checked for errors.

Furthermore, `asl_send` is used directly instead of `asl_log` to remove the level of indirection associated with `asl_log`.
I suppose `if (1 || 2 || 3) { asl_free }` works too.  Only issue is that `asl_free` needs to be called unconditionally too, once `asl_send` is called.  To me, the nice thing about the `goto` is it results in one location for resource release.

I will change it if you would prefer to avoid the gotos.