WebDAV: accept compliance class 2 or 3 (closes bitfireAT/davx5#21); fix...

WebDAV: accept compliance class 2 or 3 (closes bitfireAT/davx5#21); fix division by zero in progress

- WebDAV service detection: accept compliance class 2 or 3, because they both imply class 1 (there are servers which don't show class 1 although they MUST)
- WebDAV random access: fix division by zero when file size reported by HEAD is zero