* gc.h, gc.c (scm_gc_sweep): Issue deprecation warning when

* gc.h, gc.c (scm_gc_sweep): Issue deprecation warning when
non-zero is returned from a port or smob free function.
(scm_malloc, scm_realloc, scm_strndup, scm_strdup,
scm_gc_register_collectable_memory,
scm_gc_unregister_collectable_memory, scm_gc_malloc,
scm_gc_realloc, scm_gc_free, scm_gc_strndup, scm_gc_strdup): New.

* backtrace.c, continuations.c, convert.i.c, coop-threads.c,
debug-malloc.c, dynl.c, environments.c, environments.h,
extensions.c, filesys.c, fports.c, gc.c, gc.h, gh_data.c, goops.c,
guardians.c, hooks.c, init.c, keywords.c, load.c, numbers.c,
ports.c, posix.c, procs.c, rdelim.c, regex-posix.c, root.c,
smob.c, stime.c, strings.c, struct.c, struct.h, symbols.c, unif.c,
vectors.c, weaks.c: Use scm_gc_malloc/scm_malloc and
scm_gc_free/free instead of scm_must_malloc and scm_must_free, as
appropriate.  Return zero from smob and port free functions.

* debug-malloc.c (scm_malloc_reregister): Handle "old == NULL".

* fports.c (scm_setvbuf): Reset read buffer to saved values when
it is pointing to the putback buffer.