branch do work.

Add test.

Add test.

Introduce new MountOptions struct with AllowOther option.

archive agnostic MemTree and MemTreeFileSystem.

This allows an in-process overlay of eg. r/o ZipFs and r/w LoopbackFileSystem.

Use it in UnionFs.

Print version number in Unionfs and AutoUnionFs.

This fixes a potential null dereference.

Drop fs.Release() as FileSystem API method.

This should fix GetAttr racing with Release in UnionFs, where the
GetAttr may pickup the size from before the write.