shutdown: flush wallet.dat on Windows headless stop

The headless loop installed SIGINT/SIGTERM handlers only under #ifndef _WIN32,
so a headless node on Windows never reached the clean StopNode()+DBFlush() path:
Ctrl-C, closing the console window, a service stop and OS shutdown all killed it
mid-write, and a Berkeley DB wallet.dat left unflushed will not reopen.

Install a SetConsoleCtrlHandler that mirrors the POSIX path: set fShutdown and,
because the OS terminates us shortly after the handler returns on
CLOSE/LOGOFF/SHUTDOWN, block until the main loop signals the flush is done.
A 'taskkill /F' is still an uncatchable hard kill by design.

Closes #16.

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