CLEANUP: pools: get rid of the POOL_LINK macro

CLEANUP: pools: get rid of the POOL_LINK macro

The POOL_LINK macro is now only used for debugging, and it still requires
ifdefs around, which needlessly complicates the code. Let's replace it
and the calling code with a new pair of macros: POOL_DEBUG_SET_MARK()
and POOL_DEBUG_CHECK_MARK(), that respectively store and check the pool
pointer in the extra location at the end of the pool. This removes 4
pairs of ifdefs in the middle of the code.