fix boot program detection, which in turn makes `make-stack' actually work

fix boot program detection, which in turn makes `make-stack' actually work

* libguile/programs.h (SCM_F_PROGRAM_IS_BOOT, SCM_PROGRAM_IS_BOOT): Flags
  for determining if a program is a boot program. It turns out that our
  heuristics e.g. in stacks.c would catch non-boot programs, like
  programs that end with (goto/args 1), because the 1 is the same byte as
  `halt'. That took a while to find...

* libguile/stacks.c (stack_depth, read_frames): Use the new boot prog
  macros.
  (scm_make_stack): Assert that we read the number of frames that we said
  we would.

* libguile/vm.c (really_make_boot_program): Mark boot programs
  appropriately.