verification: ignore some filemode bits

User may want to ignore the permission bits specified in the manifest during verification, for platform-specific reason.

e.g. Create a manifest on Unix and verify it on [Windows](https://golang.org/src/os/types_windows.go#L31), and vice versa

This could be implemented as a bitmask like this
```console
$ continuity verify --ignore-mode-bits 0777
```