* gc.c: (scm_gc_mark_dependencies): new function. like

* gc.c: (scm_gc_mark_dependencies): new function.  like
`scm_gc_mark', but doesn't mark the argument itself.  defined
using an arrangement similar to that in eval.c: `scm_gc_mark' and
`scm_gc_mark_dependencies' are derived from the same "template"
by ugly preprocessor magic.

* gc.h: added prototype for `scm_gc_mark_dependencies'.

* init.c (scm_init_guile_1): call the renamed
`scm_init_guardians'.

* guardians.h: changed prototypes for `scm_make_guardian' and
`scm_init_guardians'.

* guardians.c (guardian_t): added new fields `greedy_p' and
`listed_p'.
(GUARDIAN_P): predicate that says whether its argument is a
guardian.
(GUARDIAN_GREEDY_P, GUARDIAN_LISTED_P): new predicates.
(greedy_guardians, sharing_guardians): new variables.  hold the
greedy and sharing live guardian lists, respectively.
(first_live_guardian, current_link_field): removed.
(greedily_guarded_prop): new variable.  holds the "is greedily
guarded" object property.
(self_centered_zombies): new variable.  stores guarded objects
that are parts of cycles.
(add_to_live_list): new function, introduced to decouple marking a
guardian and adding it to the live list.
(guardian_mark): call `add_to_live_list'.
(guardian_print): print whether the guardian is greedy or not.
also change "live" and "zombie" to "reachable" and "unreachable"
respectively, to be less confusing.
(scm_guard): if the guardian is greedy, test whether the object is
already greedily marked.  throw an error if so.
(scm_get_one_zombie): if the guardian is greedy, remove the
"greedily guarded" property from the object.
(scm_make_guardian): add a new optional boolean argument which
says whether the guardian is greedy or sharing.
(guardian_gc_init): init the new live lists.
(mark_dependencies): new function.
(mark_and_zombify): new function.
(guardian_zombify): reworked to support the new guardian
semantics.  move some logic to `mark_dependencies' and
`mark_and_zombify'.
(whine_about_self_centered_zombies): new function.  installed in
the `after-gc-hook' to complain about guarded objects which are
parts of cycles.
(scm_init_guardians): init the new stuff.  renamed from
`scm_init_guardian'.