* gc.c (scm_igc): Check for scm_mallocated underflow. Otherwise,

* gc.c (scm_igc): Check for scm_mallocated underflow.  Otherwise,
it shows up as terrible performance, as we GC constantly.
* gc.c (scm_init_storage): install an atexit proc to flush the
ports.
(cleanup): the new proc.  it sets a global variable which can be
checked by the ptob flush procs to avoid trying to throw
exceptions during exit.  not very pleasant but it seems more reliable.
* fports.c (local_fflush): check terminating variable and if set
don't throw exception.
* CHECKME: that the atexit proc is installed if unexec used.