Simplify smob and port marking; set the mark bit in the generic

Simplify smob and port marking; set the mark bit in the generic
marking code, and make marker routines only responsible for
turning up outgoing pointers.
* gc.c (scm_gc_mark): Set the mark bit on ports and smobs here,
before calling the marking function.  Don't call the marking
function if it's zero.
* markers.c (scm_mark0): Just return #f.  This function isn't
necessary at all now, but it's harmless to call it.  We'll leave
it in so other folks' code doesn't croak at link time.
(scm_markcdr): Don't call SCM_SETGC8MARK.
* async.c (mark_async): Don't call SCM_SETGC8MARK.
* dynl.c (mark_dynl_obj): Same.
* root.c (mark_root): Same.
* srcprop.c (marksrcprops): Same.
* unif.c (markra): Same.
* variable.c (scm_markvar): Same.
* ports.c (scm_markstream): Same.
(void_port_ptob): Specify zero for our marking function.
* debug.c (debugobjsmob): Same.
* dynwind.c (guardsmob): Same.
* filesys.c (dir_smob): Same.
* fluids.c (fluid_smob): Same.
* fports.c (scm_fptob, scm_pipob): Same.
* mallocs.c (mallocsmob): Same.
* regex-posix.c (regex_t_smob): Same.
* smob.c (freecell, flob, bigob): Same.
* threads.c (thread_smob, mutex_smob, condvar_smob): Same.
* throw.c (jbsmob, lazy_catch_funs): Same.