Honor the pax-option overrides when creating archive.

Changes proposed by Denis Excoffier.

* NEWS: Fix typos.
* doc/tar.texi: Fix typos.  Improve recipe for creation of binary
equivalent archives.
* src/create.c (write_extended): Use the value of the
--mtime option (if specified) as the default for exthdr.mtime.
* src/xheader.c (xheader_store): Create the header if at least
one override is supplied in --pax-option.