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`.
Isn't this equivalent to `if (__builtin_expect(asl_set(m, ASL_KEY_LEVEL, level_strings[aslLogLevel]), 0))`?

Also I don't like much `goto`. Probably not important though...