`Reader#each_entry` includes "pax_global_header"

[j8r]

I don't think we want to have pax_global_header considered as a normal file? I propose to ignore it.

[naqvis]

According to POSIX implementation is supposed to unpack an extended header as an ordinary file if it doesn't support them. Crystar only supports parsing and composing global headers, but does not currently support persisting the global state across files.