sysexits.h for Go

The sysexits package exports various ExitCode values that align with /usr/include/sysexits.h. For example, instead of returning os.Exit(0) or os.Exit(1), return os.Exit(sysexits.OK) and os.Exit(sysexits.Software), respectively.

See https://godoc.org/github.com/sean-/sysexits