a ustar-format archive. POSIX specifies that ustar-format archives have magic
strings consisting of "ustar<nul>", however our PAX accepts anything starting
with just the 5 "ustar" chars, and this and other version of GNU tar output
ustar archives with magic numbers of "ustar <nul>". Leave the output format
of this version of GNU tar the same, so that old versions of GNU tar can
use ustar files it outputs. (diff sent to GNU tar maintainer.)
