Disallow modifications to the global pax extended header in update mode.

Disallow modifications to the global pax extended header in update mode.

Updating global headers in update mode is not possible, because:

1) If the original archive was not in PAX format, writing the
global header would overwrite first member header (and eventually
data blocks) in the archive.
2) Otherwise, using the --pax-option can make the updated header
occupy more blocks than the original one, which would lead to the
same effect as in 1.

This also fixes
  http://lists.gnu.org/archive/html/bug-tar/2018-12/msg00007.html

* src/xheader.c (xheader_forbid_global): New function.
* src/common.h (xheader_forbid_global): New prototype.
* src/update.c (update_archive): Use xheader_forbid_global, instead
of trying to write global extended header record.