main: switch log file when switching mode
plymouthd can be run in various modes, for, e.g., boot up, shutdown, and software upgrades.
The mode plymouthd is using can be changed at runtime.
The "boot" mode keeps a log of the console messages that happen during boot up. At the moment, when changing from the "boot" mode to any other mode, the log file is kept open.
That open file can cause problems during shutdown.
This commit makes sure the log file is properly closed when the mode is changed from boot to another mode.