Read POSIX multivolume archives split at the header boundary.

* src/common.h (read_header_mode): New enum.
(read_header): Change type of the 3rd argument.
* src/list.c (read_header): Change type of the 3rd argument.
All callers updated.
* src/buffer.c (try_new_volume): Allow for volumes split at the
extended/ustar header boundary. This is against POSIX specs, but
we must be able to read such archives anyway.

* tests/multiv07.at: New test case.
* tests/Makefile.am: Add multiv07.at
* tests/testsuite.at: Likewise.

* src/compare.c: Update calls to read_header.
* src/delete.c: Likewise.
* src/update.c: Likewise.