MINOR: pools: report a replaced memory allocator instead of just malloc_trim()

MINOR: pools: report a replaced memory allocator instead of just malloc_trim()

Instead of reporting the inaccurate "malloc_trim() support" on -vv, let's
report the case where the memory allocator was actively replaced from the
one used at build time, as this is the corner case we want to be cautious
about. We also put a tainted bit when this happens so that it's possible
to detect it at run time (e.g. the user might have inherited it from an
environment variable during a reload operation).

The now unused is_trim_enabled() function was finally dropped.