Switch to labkit for logging system setup

The only user-visible portions of this are:

• We start supporting the "color" format for logs.
• We now respond to SIGHUP by reopening the log file.
• We now respect the log format when no log filename is specified.

Output to syslog in the event of logging system setup is preserved in OpenSSH mode.

Part of #499 (closed)

Closes #271 (closed)