verification: ignore some filemode bits

[AkihiroSuda]

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, and vice versa

This could be implemented as a bitmask like this

$ continuity verify --ignore-mode-bits 0777

[stevvooe]

@AkihiroSuda Isn't this just a mask?

[AkihiroSuda]

Yes, it is just a mask. Is it already implemented?

[stevvooe]

@AkihiroSuda No, but it should be straightforward.