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`.
As for `goto`:

> Albeit deprecated by some people, the equivalent of the goto statement is
used frequently by compilers in form of the unconditional jump instruction.

> The goto statement comes in handy when a function exits from multiple
locations and some common work such as cleanup has to be done.  If there is no
cleanup needed then just return directly.

Maybe in this simple cases everything could written as:

```obj-c
if (check1 ||
    check2 ||
    check3) {
    asl_free(m);
}
```
?