Initialize headers by default to EntryType::Regular

[e-nomem]

Implementation for #422

This is a breaking change to the library.

[xzfc]

There are two different changes:

1. Split EntryType::Regular into LegacyRegular and Regular.
2. Use b'0' instead of b'\0' by default in new_gnu and new_ustar.

In the wild (see grep.app) I see many uses of checks like if header.entry_type() != tar::EntryType::Regular. The first change would silently break such code, i.e. no compile error, but runtime behavior would change. Can it be designed in a non-breaking way?

The second change is less drastic, but still I'd prefer not to change the output across tar-rs versions within 0.4.x version range.

[e-nomem]

I suppose we could make EntryType::Regular into a tuple or struct variant with some information about if it's a 'legacy' regular file or not? That would break any code matching against EntryType::Regular so it doesn't silently fail.

[xzfc]

That still would be a a semver violation.

[e-nomem]

Yeah, this PR basically has to be breaking because of the change in the encoded default entry type.