MINOR: pools: partially uninline pool_alloc()

MINOR: pools: partially uninline pool_alloc()

The pool_alloc() function was already a wrapper to __pool_alloc() which
was also inlined but took a set of flags. This latter was uninlined and
moved to pool.c, and pool_alloc()/pool_zalloc() turned to macros so that
they can more easily evolve to support debugging options.

The number of call places made this code grow over time and doing only
this change saved ~1% of the whole executable's size.