cast alloc's to shut SunOS up

cast alloc's to shut SunOS up

SCCS-vsn: lib/libc/db/recno/rec_get.c 8.3
SCCS-vsn: lib/libc/db/recno/rec_utils.c 8.3
SCCS-vsn: lib/libc/db/btree/bt_close.c 8.3
SCCS-vsn: lib/libc/db/btree/bt_open.c 8.5
SCCS-vsn: lib/libc/db/btree/bt_overflow.c 8.2
SCCS-vsn: lib/libc/db/btree/bt_split.c 8.3
SCCS-vsn: lib/libc/db/btree/bt_stack.c 8.3
SCCS-vsn: lib/libc/db/btree/bt_utils.c 8.4
SCCS-vsn: lib/libc/db/hash/hash.c 8.7
SCCS-vsn: lib/libc/db/hash/hash_bigkey.c 8.2
SCCS-vsn: lib/libc/db/hash/hash_buf.c 8.2
SCCS-vsn: lib/libc/db/hash/hash_page.c 8.4
SCCS-vsn: lib/libc/db/mpool/mpool.c 8.2