Add VK_LOADER_LAYERS_ALLOW environment variable.

Add VK_LOADER_LAYERS_ALLOW environment variable.

This new environment variable prevents the VK_LOADER_LAYERS_DISABLE
env-var from applying to layers in the VK_LOADER_LAYERS_ALLOw env-var.

The purpose of it is to act like VK_LOADER_LAYERS_ENABLE in regards to
VK_LOADER_LAYER_DISABLE without force enabling the layers. That way,
implicit layers which normally would only be enabled in certain
circumstances (eg their enable_environment is set) can be allowed
while still disallowing other implicit layers.