Rework Software Expiry

[pithosian]

As an alternative to #124.

• Add a warning on startup within 4 weeks of expiry to bitcoind and bitcoin-qt.
• Add a note on how to override in the existing bitcoind startup failure message for expired software.
• Replace the existing bitcoin-qt startup error message for expired software with an input box requiring the user to type I accept this software may be unsafe. to continue (overriding expiry this way updates configuration).
• Shut down when bitcoind or bitcoin-qt expire at runtime, rather than silently rejecting blocks.

The rationale for these changes is largely explained by my comments on the above-linked PR. I considered using the same prompt at runtime as at startup for bitcoin-qt, but the churn required was much higher with minimal benefit (as it stands, the software pops up a dialog telling the user the software is expired, on closing, bitcoin-qt exits, and if the user tries to start it again, they get the override prompt).

As a brief summary:

• It's good to make a user who hasn't explicitly indicated that they want to run EOLed software (see Core's end of life policy and disclosure policy) think about what they're running.
• In bitcoind, shutting down with an error message telling the user how to override is effectively identical to pushing a prompt for continued operation when running attached to a terminal and, when not running attached to a terminal (eg disowned, or under a service manager), a prompt cannot be displayed anyway.
• bitcoin-qt now gives the user the opportunity to override without going and manually editing the configuration files, without simply showing them a dialog they can hit yes on to continue without thinking about anything.
• Shutting down outright is far more detectable and safe than leaving the node running, but silently rejecting blocks.

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