New -n/--no-header argument: don't write global archive header

Allows using shell redirection to append files to existing archive. Eg.:

    $ ptar c foo/ba* > foo.ptar
    $ cat foo.ptar | ptar t
    foo/bar
    foo/baz
    $ ptar -n c foo/buz >> foo.ptar
    $ cat foo.ptar | ptar t
    foo/bar
    foo/baz
    foo/buz