Fix omission of VM frames from backtrace

Fix omission of VM frames from backtrace

From the time when a #<program> was a SMOB, really_make_boot_program
in vm.c was still using SCM_SET_SMOB_FLAGS to set the
SCM_F_PROGRAM_IS_BOOT flag - which meant that it was setting flag
1<<32 :-) which obviously was then missed by the SCM_PROGRAM_IS_BOOT
calls in stacks.c.

* libguile/programs.h (SCM_F_PROGRAM_IS_BOOT): Use a less significant
  bit for this flag, now that programs use a tc7 type.

* libguile/vm.c (really_make_boot_program): Don't use
  SCM_SET_SMOB_FLAGS, now that programs aren't SMOBs.