Fix tar files emitted by pg_dump and pg_basebackup to be POSIX conformant.

Both programs got the "magic" string wrong, causing standard-conforming tar
implementations to believe the output was just legacy tar format without
any POSIX extensions.  This doesn't actually matter that much, especially
since pg_dump failed to fill the POSIX fields anyway, but still there is
little point in emitting tar format if we can't be compliant with the
standard.  In addition, pg_dump failed to write the EOF marker correctly
(there should be 2 blocks of zeroes not just one), pg_basebackup put the
numeric group ID in the wrong place, and both programs had a pretty
brain-dead idea of how to compute the checksum.  Fix all that and improve
the comments a bit.

pg_restore is modified to accept either the correct POSIX-compliant "magic"
string or the previous value.  This part of the change will need to be
back-patched to avoid an unnecessary compatibility break when a previous
version tries to read tar-format output from 9.3 pg_dump.

Brian Weaver and Tom Lane

doc/src/sgml/protocol.sgml

@@ -1759,9 +1759,11 @@ The commands accepted in walsender mode are:
       After the second regular result set, one or more CopyResponse results
       will be sent, one for PGDATA and one for each additional tablespace other
       than <literal>pg_default</> and <literal>pg_global</>. The data in
-      the CopyResponse results will be a tar format (using ustar00
-      extensions) dump of the tablespace contents. After the tar data is
-      complete, a final ordinary result set will be sent.
+      the CopyResponse results will be a tar format (following the
+      <quote>ustar interchange format</> specified in the POSIX 1003.1-2008
+      standard) dump of the tablespace contents, except that the two trailing
+      blocks of zeroes specified in the standard are omitted.
+      After the tar data is complete, a final ordinary result set will be sent.
      </para>
 
      <para>