appserver/client cannot use print(), so use stdout.write instead
appserver/client cannot use print(), so use stdout.write instead

`flappclient run-command` is used to connect to a remote process, like ssh,
so we might be transferring arbitrary binary data over stdin/out/err. To do
this in py3, we need to use e.g. `sys.stdout.buffer` instead of plain
`sys.stdout` (which only accepts text/unicode). When testing this, we use a
BytesIO for stdout/stderr, rather than a StringIO.

In py3, print(file=BytesIO) doesn't work: print() always turns its payload
into text first, and you can't write text to a BytesIO.

So we replace all print(file=options.stdout) with options.stdout.write() .

warner/foolscap