Removes pistol restriction from holster

This allows for any gun ( within holster volume limitation [ currently 5 for normal and shoulder holster and 3 for ankle] ) to be holstered and unholstered using said guns default skill requirement. ( Meaning to holster a pistol it would check your pistol skill, shotgun checks your shotgun skill, etc. ) This also renames the HOLSTER_PISTOL iuse flag to HOLSTER_GUN.

I haven't added any larger holsters yet, so currently this will only allow the player to holster a sawn-off shotgun. This is subject to change though.

Issue: #10515 
doesn't this need to call `&iuse::holster_gun` as well?