stdout/stderr around
A lot of functions in the cmd and ctl packages were passing these
around and barely using them. Replaced them with a logger for most
functions. Some functions get an io.Writer instead so that their
tests can find the output they're looking for.
More cleanup on fb-1766: reworked the tests that were using io.Pipe
or os.Pipe to check their results so they now use a bytes.Buffer.
Unexported some variables that didn't need to be exported.
Fixed NewConfigCommand to use the provided stderr, not os.Stderr.
Added tests for rbf_dump, rbf_page, and keygen, since those weren't
being tested at all.
Added chksum_test, final cleanup.
Cobra automatically displays usage messages, and also a gratuitous
"Error: [...]" line in some cases, when any error at all occurs
running a command. To suppress the usage message, you have to set
cmd.SilenceUsage to true. But the code that would do this doesn't
have access to it. To address this, we introduce a category of
"usage error", implemented with stdlib error wrapping (%w) and
use errors.Is to check for it. There's also utility functions
to do this checking automatically, or indeed, to handle wrapping
of the ctl.SomethingCommand and handle running it with a suitable
context and everything.
In fact, several of the places we're checking for usage errors,
we can never actually report one, but we're checking consistently
so that if we want to report usage errors, we can.
For instance, server.Start and (dax)server.Start don't ever
return usage errors, right now, but we're checking their responses
anyway.
i used this script, a little clunky but it got the job done
```bash
for file in `find . -type f -print | grep '\.go'`; do
sed '1,/^\/\/ limitations under the License.$/d' $file > $file.tmp;
result=`cat $file.tmp`
if [[ result != "" ]]; then
gofmt $file.tmp &> /dev/null;
if [[ $? == 0 ]]; then
mv $file.tmp $file && gofmt -w $file;
else
rm $file.tmp;
fi
else
rm $file.tmp;
fi
done
```
