util: Add SignalInterrupt class and use in shutdown.cpp

Summary:
This change helps generalize shutdown code so an interrupt can be
provided to libbitcoinkernel callers. This may also be useful to
eventually de-globalize all of the shutdown code.

Co-authored-by: Russell Yanofsky <russ@yanofsky.org>
Co-authored-by: TheCharlatan <seb.kung@gmail.com>

This is a partial backport of core#27861
https://github.com/bitcoin/bitcoin/pull/27861/commits/e2d680a32d757de0ef8eb836047a0daa1d82e3c4
Depends on D18964

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18965