unvquery: all non `-` slot in `B` key is bot

See:

• bot: store bot skill and generation status in slotBots B cvar and display it on scoreboard (reuse ping cell) github.com:Unvanquished/Unvanquished!2437

The idea is to use the B key to store various bot status: navmesh being generated, bot skill…

So every B slot that is not - would be a bot.