Correctly detect 'pax' format archives in append mode

Correctly detect 'pax' format archives in append mode

We expect that existing pax archives start with a global or extended
header. If they don't, append operations will be done using ustar
format.

Fixes append mode on pax archives where pax(1) would bail out when
appending to pax archives, falsely detecting a mismatch.  Reading was
unaffected.  Reported by caspar@, ok caspar@ millert@