Set log path from commandline

[ahuillet]

Hello,

the Xpra log is hardcoded to go in ~/.xpra/xpra-$DISPLAY.log.

It may be useful to be able to tell the server, on the commandline, where to store the log:

xpra start --log-file /path/to/log.log

would be great.

Thanks

[Antoine Martin]

dupe of #213