goshs: WebDAV listener ignores --read-only, --upload-only, and --no-delete mode flags

goshs is a SimpleHTTPServer written in Go. Prior to version 2.1.0, when `goshs` is launched with WebDAV enabled (`-w`), the mode-restriction flags `--read-only`, `--upload-only`, and `--no-delete` are enforced only on the primary HTTP port. The WebDAV port is wired straight to `golang.org/x/net/webdav.Handler` with no equivalent guard, so an authenticated WebDAV client can `PUT`, `DELETE`, `MKCOL`, `MOVE`, and `COPY` despite the operator's stated intent. Version 2.1.0 patches the issue.