Lower max value for "guard-extreme-restriction-percent" to 100%

See torspec#66 (closed). I fixed this with 98cf7ede083dc0fbb7242c7e557a8a2d5499e135 in torspec, but we may as well do it in Tor too.

This change will have no effect in practice; setting a value of (say) 200 here would have meant "always warn", just the same as 100 will mean now.