Adjust spawn protection dedicated server option to allow use of invuln powerup

[GooberRF]

This PR adjusts the implementation of spawn protection via the dedicated server config file in two ways:

• It is now a boolean on/off toggle with a duration optional parameter. This is very minor, but I think it is more intuitive than the previous method and aligns better with other options with configurable parameters like hitsounds.
• It now has a new $Use Powerup optional setting (defaults false). If true, instead of using the standard method of spawn protection, grant newly spawned players an invulnerability powerup for the specified duration. This probably would never be used in normal servers but it would be fantastic for run servers. It's effectively what Cyrus and many other mappers were intending with all the invulnerability powerups scattered around.

New syntax for the configuration is (with default values shown):

$DF Spawn Protection Enabled: true
    +Duration: 1500
    +Use Powerup: false