qt: clean up initialize/shutdown signals

Summary:

• Change initializeResult(int) to initializeResult(bool) to avoid implicit type conversion.
• Use EXIT_FAILURE and EXIT_SUCCESS instead of magic numbers.
• Remove the argument from shutdownResult(int); it was called with a constant argument.

Backport of https://github.com/bitcoin/bitcoin/pull/9834

Completes T427

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: make check && test_runner.py

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1897