lockedpool: avoid sensitive data in core files (FreeBSD)
lockedpool: avoid sensitive data in core files (FreeBSD)

This is a followup to
23991ee53 / https://github.com/bitcoin/bitcoin/pull/15600
to also use madvise(2) on FreeBSD to avoid sensitive data allocated
with secure_allocator ending up in core files in addition to preventing
it from going to the swap.

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