(flush_write,flush_read): Change data type.

(flush_archive): Compute actual buffer fill level before calling
low level function.
(close_archive): Call flush_archive again if the first call
resulted in partially filled buffer.
(try_new_volume): Rewritten handling of initial headers.
(add_chunk_header): New function. Write an additional header
before the continuation chunk. The purpose of the header is to
allow third-party tars to extract the member.
(simple_flush_write): Take an argument.
(_gnu_flush_write): Correctly handle partially filled buffers.