Add a readUID property to DDASLLogger

Set readUID to -1 to allow any user to read the ASL log entry.
By default, readUID == -1.
Match NSLog readUID functionality in DDASLLogger

NSLog sets the read UID property of ASL log messages to the current euid.

Also, use asl_send instead of asl_log to skip a level of indirection.
Add error handling to asl(3) calls
Increase code readability
Rename variable for readability
Remove AssertMacros dependence
Remove __builtin_expect and goto usage
Merge pull request #339 from slarew/readuid_property

Match NSLog read UID functionality in DDASLLogger