Merge #13163: Make it clear which functions that are intended to be translation unit local (bitcoind.cpp)

Summary:
c3f34d06be Make it clear which functions that are intended to be translation unit local (practicalswift)

Pull request description:

Make it clear which functions that are intended to be translation unit local.

Do not share functions that are meant to be translation unit local with other translation units. Use internal linkage for those consistently.

Tree-SHA512: 05eebd233d5cfbf6116724eec3a99b465bf534ca220f2b6f5e56341a7da41387454d3cb6ceadd8ab6714a5df94069e4ad0dcab8801ccc7e8949be7199a19fb53

Partial Backport of Core PR13163 (bitcoind.cpp)
https://github.com/bitcoin/bitcoin/pull/13163/

Part of breaking up D3293

Test Plan:

make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox, deadalnix, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3524