Allow loading mod maps only

[L-P]

Add sv_load_mod_maps, contrary to sv_load_all_maps it only loads
mod-specific maps to avoid loading 125 extra maps if you base your mod on
valve and only care about your own maps.

BC break: for it to make sense I had to rename sv_stop_loading_all_maps to
sv_stop_loading_maps. I could add a sv_stop_loading_mod_maps instead.

[JoelTroch]

Personally, I think it would be best to remove the "all" version of the command (the ones that grabs HL1 maps) and only keep the "mod" one.

The only use case for a custom mod where I see re-building HL1 maps node graphs as a necessity would be that said mod did a change in the node graph version/structure (or use something completely different) and re-use the HL1 BSPs "as is". That makes it a very "niche" case considering that mods messing around with the node graph system (and re-using HL1 BSPs) isn't something you see every day if I can say it that way.

If anyone else has thoughts on the matter, feel free to share.