[#2335] [GTKUI] Fix startup failing with 'cannot acquire lock'

This issue was caused by an unclean shutdown of Deluge, usually on system shutdown, and upon rebooting
the PID stored in the lockfile is in used by another process thus the lockfile is never removed. It
affects users with Deluge set in startup applications as the PIDs are more likely to be reused.

 * Lockfile is removed if Deluge is restarted in IPC.
 * Renamed the old_tempfile variable to make it clearer as to it's role.