Allow manual powerup dropping (with use key) & fix powerup dropping outside of the map

This MR:

• Adds the ability to manually drop a powerup with the use key (with g_powerups_drop cvar enabled)
• It fixes a bug where powerups sometimes spawned outside of the map when using g_powerups_drop_ondeath.

Same as drop_ondeath, it can be set to either drop with a frozen timer, or a running timer (like in the video):

strength