629a47a154 Document FreeBSD quirk. Fix FreeBSD build. (practicalswift)
Pull request description:
* Document FreeBSD quirk.
* Fix FreeBSD build: Cast to `int` to allow `std::min` to work under FreeBSD.
Context: https://github.com/bitcoin/bitcoin/pull/9598#issuecomment-398353904
Tree-SHA512: 5ca7a5fa9e1f3efae241b9be64c9b019ec713c11dcc3edaaed383477ea48ac0dc82549ffebbe9069e8c3f6eff30acd6e4542b4aa31d307f022f4f51e5851a82c
Backport of Core PR13503