Code refactoring of GC_remove_from_fl

Code refactoring of GC_remove_from_fl

* allchblk.c (INCR_FREE_BYTES, FREE_ASSERT, FL_UNKNOWN): Remove.
* allchblk.c (PHDR, NHDR): Enclose argument in parentheses.
* allchblk.c (IS_MAPPED): Replace 1 with TRUE.
* allchblk.c (GC_remove_from_fl, GC_add_to_fl, GC_split_block): Expand
FREE_ASSERT and INCR_FREE_BYTES macros.
* allchblk.c (GC_remove_from_fl): Decouple to GC_remove_from_fl_at
(for known free list index) and to single-argument GC_remove_from_fl;
update the comment.
* allchblk.c (GC_merge_unmapped, GC_get_first_part, GC_allochblk_nth,
GC_freehblk): Adjust GC_remove_from_fl invocation.
* allchblk.c (GC_split_block): Make assertion on GC_free_bytes element
stricter (check for overflow).
* allchblk.c (GC_free_block_ending_at, GC_add_to_fl): Reformat comment.