journald: enable persistent FD Store to fix logging during soft-reboot

journald: enable persistent FD Store to fix logging during soft-reboot

A unit with StandardOutput=journal (the default) will get its stdout/stderr sockets
disconnected when journald stops, as the file descriptors on journald's side are
not preserved (it works on restart, as the FD Store keeps them open during restarts).
Set FileDescriptorStorePreserve=yes so that the journal FD's stay open during a soft
reboot, and applications don't get broken stdout/stderr.