sync registers before calling c functions, really

sync registers before calling c functions, really

* src/vm_loader.c:
* src/vm_scheme.c: Use SYNC_REGISTER, although it's the same as
  SYNC_BEFORE_GC. A style issue?

* src/vm_system.c (call, tail-call): Explicitly do a SYNC_REGISTER, not
  relying on POP_LIST to do it for us, because POP_LIST won't do it if
  there are 0 arguments. The way that this manifested itself to me was
  badness after the second (read) call in a repl session. I wish I
  understood more about this problem.