SCons: Streamline Vulkan buildsystem + fixups

- Renamed option to `builtin_vulkan`, since that's the name of the
  library and if we were to add new components, we'd likely use that
  same option.
- Merge `vulkan_loader/SCsub` in `vulkan/SCsub`.
- Accordingly, don't use built-in Vulkan headers when not building
  against the built-in loader library.
- Drop Vulkan registry which we don't appear to need currently.
- Style and permission fixes.

drivers/SCsub

@@ -22,9 +22,6 @@ SConscript('alsamidi/SCsub')
 SConscript('coremidi/SCsub')
 SConscript('winmidi/SCsub')
 
-if env["builtin_vulkan_loader"] and not (env["platform"]=="osx" and env['use_static_mvk']):
-    SConscript('vulkan_loader/SCsub')
-
 # Graphics drivers
 if (env["platform"] != "server"):
 #    SConscript('gles3/SCsub')