(xdup2): Don't report errno if dup returns an unexpected nonnegative value.
(open_archive): Reject multivolume verify attempts a bit earlier.
Rename local variable `access', in case it's defined by system header.
(open_archive, backspace_output): Use `Cannot' uniformly, instead of
`Could not' sometimes and `Cannot' others.
(open_archive, flush_read, flush_archive, close_archive, new_volume):
Quote arbitrary strings in diagnostics.
(read_error): Set archive to STDOUT_FILENO temporarily when writing
archive buffer.
(init_volume_number): Check for input and output errors in volno_file.
(new_volume): Use new fatal_exit function to exit, and new xfork
function to fork.