Fix a bug in multi-volume archive creation.

When creating multivolume archives, the bufmap code in buffer.c
implicitly assumed that the members are stored in the archive
contiguously, ignoring the member (and eventual extended) headers
between them.  This worked until the member header happened to be
at the very beginning of the volume, in which case its length was
included in the calculation of the stored size and size left to
store.  Due to this, the GNUFileParts extended header contained
invalid GNU.volume.offset value, and the resulting archive failed
to extract properly.

This patch also eliminates improper listing of file part headers
as regular files, when creating multivolume posix archives with -v.

* src/buffer.c (bufmap): New member nblocks.  Counts number of blocks
of file data written since reset.
(bufmap_reset): Reset nblocks to 0.
(_flush_write): Update nblocks.  When computing offset difference for
bufmap_reset, count only data blocks, not headers.
(close_archive): Flush archive until all blocks are written.
(add_chunk_header): Use simple_finish_header instead of finish_header
to avoid listing chunk header as regular file in verbose mode.
* tests/multiv10.at: New test case.
* tests/Makefile.am: Add new test.
* tests/testsuite.at: Add new test.

tests/testsuite.at

@@ -377,6 +377,7 @@ m4_include([multiv06.at])
 m4_include([multiv07.at])
 m4_include([multiv08.at])
 m4_include([multiv09.at])
+m4_include([multiv10.at])
 
 AT_BANNER([Owner and Groups])
 m4_include([owner.at])