Decode the arguments passed to the *at() family of system calls. This is
especially useful now that libc's open() always calls openat(). While here,
fix a few other things:

• Decode the mode argument passed to access(), eaccess(), and faccessat().
• Decode the atfd paramete to pretty-print AT_FDCWD.
• Decode the special AT_* flags used with some of the *at() system calls.
• Decode arguments for fchmod(), lchmod(), fchown(), lchown(), eaccess(), and futimens().
• Decode both of the timeval structures passed to futimes() instead of just the first one.