test --logfile and --stderrlog command-line options

- --logfile /tmp/test.log writes to a file
- --logfile '' (empty) disables file logging, falls back to syslog
- --stderrlog writes to stderr
- config logfile= overridden by command-line --logfile
- --stderrlog + --logfile together: stderr wins, logfile is not written
- reverse ordering tests: config overrides empty --logfile, config-first with stderrlog
