Improve GC_allochblk algorithm of block splitting when unmapping enabled

Improve GC_allochblk algorithm of block splitting when unmapping enabled
(allow splitting up to limit based on per-list free space amount but
skipping unmapped blocks)

* allchblk.c (GC_allochblk_nth): Change type of "may_split" argument
from GC_bool to int (to hold 3rd value).
* allchblk.c (AVOID_SPLIT_REMAPPED): New macro (used as a value for
"may_split").
* allchblk.c (GC_allochblk): Add "may_split" local variable (set to
TRUE by default and passed to GC_allochblk_nth); do not zero
split_limit in case of USE_MUNMAP (and remove FIXME); set may_split to
AVOID_SPLIT_REMAPPED in case of USE_MUNMAP and split_limit is
a non-zero value computed by GC_enough_large_bytes_left.
* allchblk.c (GC_allochblk_nth): Update usage comment; if may_split is
AVOID_SPLIT_REMAPPED and hhdr is unmapped then continue search instead
of remapping memory and splitting the block.