Some minor tweaks to the QC physics logic

Adds support for the QC based .move_nomonsters collision field to sv_gameplayfix_downtracesupportsongroundflag. One of the parameters of _Movetype_PushEntity was always true so I've removed it to boost performance ever so slightly.
A slow falling fix that normalized a vector was using an unwrapped version (unsure why!), so I've switched it to use the standard normalize builtin for another tiny performance boost.
Also renamed some iterator variables for readability.