upstream: Rework logging for the regression tests.

upstream: Rework logging for the regression tests.

Previously we would log to ssh.log and sshd.log, but that is insufficient
for tests that have more than one concurent ssh/sshd.

Instead, we'll log to separate datestamped files in a $OBJ/log/ and
leave a symlink at the previous location pointing at the most recent
instance with an entry in regress.log showing which files were created
at each point.  This should be sufficient to reconstruct what happened
even for tests that use multiple instances of each program.  If the test
fails, tar up all of the logs for later analysis.

This will let us also capture the output from some of the other tools
which was previously sent to /dev/null although most of those will be
in future commits.

OpenBSD-Regress-ID: f802aa9e7fa51d1a01225c05fb0412d015c33e24