main: make /gen actually mine (#42)

nMineMode defaults to MINE_RELAY (0) and ParseStartupArguments() never changed
it, so a node started with /gen would spawn ThreadBitcoinMiner, enter
BitcoinMiner(), hit

    if (nMineMode == MINE_RELAY)
        return true;

and return before doing any work. The node then ran indefinitely looking
completely healthy -- it synced, it held peers, it reported no errors -- while
earning nothing. Nothing in the log said otherwise; the only hint was
`nMineMode = 0` in a startup line that reads like a status echo.

This affects every headless miner, because the GUI is the only other way to
select a mining mode.

/gen now implies MINE_SOLO unless /operator or /participant asked for something
else. Verify with `nMineMode = 1` at startup.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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