(xclose, xdup2, child_open_for_compress, child_open_for_uncompress,

(xclose, xdup2, child_open_for_compress, child_open_for_uncompress,
archive_write_error, archive_read_error, flush_archive, close_archive,
init_volume_number, new_volume):
Don't assume that gettext preserves errno.

(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.