Blender Git Loki

Fix T48487: Physics cache names allow (back)slashes, which breaks its handling.

For now, just validate new pointcache names as if they were filnames.

Ideal solution would be rather to validate names in `ptcache_filename()`, but it would
likely break some existing caches - and we'd also have to ensure we still have
uniqueness of name after it has been cleaned up, wich would not be trivial at this stage.

So let's go for simple solution for now, especially since this part of code is to be
fully rewritten in 2.8...