ply-terminal-session: Not use grantpt when the system shut down. to avoid sytem hung up.

ply-terminal-session: Not use "grantpt" when the system shut down. If we don't do this when the system has a separate /var partition, it will fail to unmount /var, the shutdown time will be very long, and sometimes it leads the system to check the disk at the next boot. and since the Plymouth only can be operated by root, the privilege has already been granted, and this has been checked in main() function.