Fix some issues and avoid warnings in disclaim-related code.

Fix some issues and avoid warnings in disclaim-related code.

* disclaim.c, include/gc_disclaim.h: Fix prototype.
* include/private/gc_priv.h, mark.c: Avoid missing initializer warning.
* mark.c: Fix type of a size_t variable in GC_reclaim_block.
* misc.c: Initialize ok_mark_unconditionally.
* reclaim.c: Tweak and avoid unused label warning.
* tests/disclaim_bench.c, tests/disclaim_test.c: Remove or
conditionalize unused variables.