Add critical hits to multiplayer

[GooberRF]

This PR adds critical hits to Dash servers optionally (default is off). This logic is inspired by Team Fortress 2, though it works a little differently. I wrote this feature in order to add a fun twist to the gameplay in the FactionFiles DM pub and it's generally been well received, so I thought I might as well PR it over so other server operators can try it out too if they wish.

Here's an explanation of the logic with default values:

• When you deal damage to an opponent, you have a 10% chance of it being a critical hit.
• Critical hits deal 4x damage AND grant you a damage amp for 1.5 seconds.
• Your chance of getting a critical hit goes up based on the amount of damage you've dealt since your last death. The bonus chance is +10% if you've dealt at least 1200 damage since your last death, which boosts your chance of a critical hit to 20%.
• When you've dealt 0 - 1200 damage since your last death, the bonus scales based on that standard. For example, if you've dealt 600 damage, your chance of a critical hit is 15%. If you've dealt 300, it's 12.5%, etc.

Configurable items in dedicated_server.txt are:

    // Enable critical hits
    $DF Critical Hits: false
        // Sound used for hit notification
        +Sound ID: 35
        // Max sound packets per second - keep it low to save bandwidth
        +Rate Limit: 10
        // Duration of damage amp reward on a critical hit
        +Reward Duration: 1500
        // Percentage chance of a critical hit
        +Base Chance Percent: 0.1
        // Enable dynamic chance bonus based on damage dealt in current life
        +Use Dynamic Chance Bonus: true
        // Amount of damage to deal in current life for the max dynamic chance bonus (+ 0.1)
        +Dynamic Chance Damage Ceiling: 1200

Note: This PR does use the same RNG code as #271, but it is identical, so there should be no conflict in merging both of them.