Added server variable sv_mountyslappychance, which can be used to make the foot slapping sounds more sporadic.

Every footstep will check a random number against sv_mountyslappychance, to see if it should slap.
By default, this has a value of 1.0, so all footsteps will slap (sv_mountyslappyfeet dependent).

This gives us justification for a new setting for sv_mountyslappyfeet, 3.
At setting 3, a default footstep will only be played if the slap does not.
This is in contrast to 1, where every footstep plays the default sound.

Feedback said slapping foot sounds were too loud, enough to cause headaches.

Updated so that slappy footsteps can be controlled with server variables.
This moved from the sound.Add system to the util.PrecacheSound method, so that volume can be dynamic.

Set sv_mountyslappyvolume to a value from 0 to 1 to adjust volume of footslaps from 0% to 100%

Default 0.5, i.e. 50% original volume

None Provided