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).