Change to minijail seccomp policy validator

[Nicole Anderson-Au]

Keep reading if you use minijail seccomp policies

This change added validation logic to the minijail seccomp policy parser that ensures that seccomp policy files and included files do not redefine syscalls. E.g. each syscall is defined only once in each file and no syscall policy is defined that is already defined in an included seccomp policy file. This feature is currently only warning and not failing on Android, but will cause seccomp parsing failures on Chrome OS. Please direct any questions to nvaa@ and jorgelo@.

Thanks,
Nicole