lib/libstand: Remove exit declaration from stand.h.

lib/libstand: Remove exit declaration from stand.h.

* Use panic instead of printf+exit in __assert().

* In the UEFI bootloader, exit will be defined as
  void exit(EFI_STATUS status);
  where EFI_STATUS is a 64bit value. So we should avoid declaring exit()
  in stand.h, since it's not used in libstand anyway.

* All exit usages in sys/boot/common and sys/boot/pc32 already declare
  their own exit() prototype.