Added shellstay and shellnum cvars.

Shells will hang around until they have lived for shellstay seconds
or until shellnum has been reached. To turn off this feature,
set shellstay to 0.
Default values are:
shellnum 16 // Number of shells allowed after being ejected
shellstay 8 // Seconds for a shell to stay after initial ejection time has passed

Fix shells getting stuck/playing multiple sounds when touching player.