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 
Good catch, both on the issue and for speaking up.  I had no notice those commits happened and the version I was testing for merge doesn't include 'em.

Back holsters shouldn't be eligible for quick-drawing, and Kevin may or may not have a problem with 'em.  I know they were an option for long guns for a while, but IIRC he blocked 'em in favor of waiting for a proper sling.