commandserver: add config knob for various logging options The default rotating options are copied from the blackbox extension.

commandserver: expand log path for convenience This allows us to set the log path relative to $XDG_RUNTIME_DIR, for instance. [cmdserver] log = $XDG_RUNTIME_DIR/chg/server.log

commandserver: switch logging facility to ui.log() interface The "pager subcommand" message is removed since ui isn't accessible there. I think that's okay as cmdtable[cmd]() will call attachio() and some debug message will be printed.

commandserver: install logger to record server events through canonical API The global commandserver.log() will be replaced with this.

commandserver: enable logging when server process started This allows us to keep track of server events before client connects to the server. Tests will be added later. Currently there's no log() call to check if things are working well.

test-commandserver: change way of triggering early crash Future patches will move the logging facility out of the server class, so cmdserver.log can't be (ab)used for this purpose. Instead, let's hook the factory function to raise exception.

loggingutil: add basic logger backends These classes will be used in command server. They are similar to the blackboxlogger, but it can't be factored out since the blackbox is so tightly coupled with a repo object.