Don't open files that will be skipped

Don't open files that will be skipped

Previously, when creating an archive file with pax(1), pax will attempt
to open a file even if the file will be skipped due to an -s replacement
with the empty string. With this change, pax will not attempt to open
files that it knows will be skipped.

When doing direct copies to a directory (-rw), pax already skips
the file before attempting to open it. So this makes the behavior
more consistent.

This can measurably speed up pax when skipping a large number of files.

OK tb@