refactor: add kernel/cs_main.*

Summary:
No more:

// Actually declared in validation.cpp; can't include because of circular dependency.
extern RecursiveMutex cs_main;

Note that including validation.h still implies that cs_main is available. In the long term we can expect cs_main to become a validation specific lock. No need to add the new include in all source files already including validation.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is a backport of core#26521
Depends on D15790

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15792