Use iobuf buffer size for temporary buffer size

* common/iobuf.c (iobuf_copy): Use iobuf buffer size for temporary
buffers.
* g10/plaintext.c (handle_plaintext, do_hash): Likewise.
* g10/sign.c (sign_file): Likewise.

As iobuf will have zerocopy operation for read/write, it is better to
use same size buffers as iobuf for temporary copy buffers.

• GnuPG-bug-id: T5828
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>