Remove barrett_diagram file duplicated by tree.html

* Makefile.direct (DOC_FILES): Remove doc/barrett_diagram entry.
* Makefile.dj (OTHER_FILES): Likewise.
* doc/doc.am (dist_pkgdata_DATA): Likewise.
* doc/barrett_diagram: Remove file (as the content is duplicated in
doc/tree.html).
Remove non-existing DISCARD_WORDS from GC data structure ASCII diagram

* doc/tree.html (picture): Remove DISCARD_WORDS (since always zero
and already removed from the code in gc-6.0alpha6).
Remove 'opp' local variable in GC_malloc_X
(code refactoring)

* alloc.c: Update comment (about free lists).
* fnlz_mlc.c (GC_finalized_malloc): Remove "opp" local variable
(replace *opp with <kind>freelist[lg]).
* gcj_mlc.c (GC_gcj_malloc, GC_gcj_malloc_ignore_off_page): Likewise.
* malloc.c (GC_malloc_atomic, GC_malloc, GC_malloc_uncollectable):
Likewise.
* mallocx.c (GC_malloc_atomic_uncollectable): Likewise.
* typd_mlc.c (GC_malloc_explicitly_typed,
GC_malloc_explicitly_typed_ignore_off_page,
GC_calloc_explicitly_typed): Likewise.
Fix ok_init assignment (missing cast) in GC_new_kind_inner

* misc.c (GC_new_kind_inner): Add cast from int to GC_bool for ok_init.
Eliminate redundant *flh check for null in GC_allocobj
(code refactoring)

* alloc.c (GC_allocobj): Do not check *flh twice if the value
is non-NULL (GC_new_hblk is not called thus *flh stays non-NULL in
that case).
Fix typo in GC_collect_or_expand comment

* alloc.c (GC_collect_or_expand): Fix a typo in comment ("setting").
Make heap growth more conservative after GC_gcollect_and_unmap call
(Consider heap size growth since latest GC_gcollect_and_unmap
invocation instead of absolute heap size value when computing amount
of extra memory blocks to add to heap in GC_collect_or_expand.)

* alloc.c (GC_heapsize_at_forced_unmap): New static variable.
* alloc.c (GC_gcollect_and_unmap): Store current heap size to
GC_heapsize_at_forced_unmap; add comment.
* alloc.c (GC_collect_or_expand): Change blocks_to_get computation
considering heap size growth since latest GC_heapsize_at_forced_unmap
call (instead of the absolute heap size value); do not call
GC_expand_hp_inner twice with same argument.
* include/private/gc_priv.h (_GC_arrays._heapsize): Refine comment.