core: make sure PrivateDevices= makes /dev/log available

Now that we moved the actual syslog socket to
/run/systemd/journal/dev-log we can actually make /dev/log a symlink to
it, when PrivateDevices= is used, thus making syslog available to
services using PrivateDevices=.
core: provide /dev/ptmx as symlink in PrivateDevices= execution environments
update TODO