Fix various USI issues

[nhamil]

This fixes a few issues with USI engines:

• During correspondence games, movetime was being provided as a number of seconds instead of milliseconds
• USI variant was not being set when the bot was started midgame, causing all midgame matches to be treated as if they were standard shogi
• Identification lines other than Fairy-Stockfish's would cause a warning

Additionally, I added checkshogi as a commented-out variant option so users are more aware that it is supported.

Please let me know if you'd like anything changed!