Move should-be-in-system-headers-but-aren't definitions from util.h to their own kernel_supplement.h.
Replace SYSCALL_FAILED/SYSCALL_MAY_RESTART macros with member functions in Registers.
Remove unneeded MAX_SYSCALLNO.

We allow recording and replay of unknown syscalls as long as they return
ENOSYS during recording.
Move SIGSTOP_SYSCALL out to where it's used.
Use std::min/max instead of MIN/MAX macros.
Move REPLAY_DESCHED_EVENT_FD out to where it's used.
Replace ALEN macro with array_length template.