tweaks for printing programs

tweaks for printing programs

* module/system/vm/program.scm (program-bindings-as-lambda-list): Handle
  the bindings-is-null case too -- not sure how it comes about, though. A
  thunk with no let, perhaps.
  (write-program): Another default for the name: the source location at
  which it was defined.

* libguile/programs.c (program_print): Add some "logic" to stop doing
  detailed prints if one print had a nonlocal exit -- preventing
  exceptions in backtraces.