Dashmode fixes

1. Players with SF_DASHMODE had their normalspeed and jumpfactor defaulted each tic while they were not in dashmode. These stats are now only reset when a) dashmode tips from above the threshold into below the threshold, or b) when the player respawns.

2. When dashmode culminates, Metal Sonic's followmobj spawns a ghost trail - this is in fact a bug, and occurs for every character with a followmobj during dashmode. This is now fixed, and the effect has been duplicated specifically for Metal's jet fume.