lockedpool: avoid sensitive data in core files (FreeBSD)

Summary:

This is a followup to PR15600 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.

This is a backport of Core PR18443

Depends on D8879

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8880