Merge #13503: Document FreeBSD quirk. Fix FreeBSD build: Use std::min<int>(...) to allow for compilation under certain FreeBSD versions.

Summary:
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

Test Plan:

ninja
ninja check-all

Repeat with FreeBSD build.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6481