* symbols.c (scm_sysintern0): New function. Contains the core of

* symbols.c (scm_sysintern0): New function.  Contains the core of
old scm_sysintern but doesn't take a second value argument.
(scm_sysintern): Now uses scm_sysintern0.
(scm_sysintern_no_module_lookup): Renamed to
scm_sysintern0_no_module_lookup and doesn't take a second value
argument any longer.

* symbols.h (scm_sysintern0: Added declaration.

* options.c (scm_init_opts): Use scm_sysintern0 instead of
scm_sysintern when interning option keys.  Otherwise we risk
destroying the values of already interned variables.

* symbols.c (scm_sym2vcell): Bugfix: Treat definedp as
scheme-level boolean (use SCM_NFALSEP).

* backtrace.c (scm_init_backtrace): Make Scheme-level variable
`the-last-stack'.
(scm_backtrace): New function. (C version of old function from
boot-9.scm) Motivation: Make it possible to display backtraces
without depending on boot-9.scm.  (I'm uncertain if this
motivation is good enough...)

* root.h (scm_root_state): Add member the_last_stack_var.
(scm_the_stack_var): Defined to scm_root->the_last_stack_var.

* root.c (mark_root): Mark scm_the_last_stack_var.

* init.c (scm_start_stack): Initialize scm_the_last_stack_var to
SCM_BOOL_F.