- Decode the arguments for several signal-related system calls: sigpending,

- Decode the arguments for several signal-related system calls: sigpending,
  sigqueue, sigreturn, sigsuspend, sigtimedwait, sigwait, sigwaitinfo, and
  thr_kill.
- Print signal sets as a structure (with {}'s) and in particular use this to
  differentiate empty sets from a NULL pointer.
- Decode arguments for some other system calls: issetugid, pipe2, sysarch
  (operations are only decoded for amd64 and i386), and thr_self.