Remove unecessary EXTEND()s from pp_sys.c

Remove unecessary EXTEND()s from pp_sys.c

If we've just POP*ed something from the stack, it will already have space.
If we conditionally POP* something from the stack, avoid the EXTEND() in that
path. Also, use PUSH() rather than XPUSH() where we know that the stack is large
enough. Changes to pp_close, pp_binmode, pp_enterwrite, pp_readlink and pp_alarm.