pax: Clear arcn in each read function.

[des]

Summary

Instead of initializing individual fields to zero, clear the entire
struct prior to populating it.

MFC after: 1 week
Sponsored by: Klara, Inc.

[markj]

bin/pax/cpio.c

277

Same below.

bin/pax/tar.c

734

Here we are writing to the arcn->name buffer. To match the old behaviour we should clear it again after this point, no?

[markj]

Looks ok aside from the style nits.

bin/pax/tar.c

734

Oh, never mind, I'm mixing up arcn->name and arcn->ln_name.

[0mp]

bin/pax/tar.c

734

But in the old version of this file we do not clear the arcn->name after this point. Am I missing something?