tinydm-run-session: back up previous log file so it's not overwritten
Sometimes the DE crashes, causing tinydm to immediately restart. When this happens, any useful info in the log file is overwritten and lost forever. This change causes tinydm to save the log file from the previous session. It only saves N-1 logs.
This behavior is similar to what X11 does with Xorg.0.log and Xorg.0.log.old.