build-sys: force creation of symlink

For some reason make sometimes wants to rerun this, and
ln would refuse to recreate the link.
systemd-python: refuse path and flags together in __init__

It's better to explictly check, instead of just documenting it.

The return value from init is changed from 1 to -1 on error.
Python seems to ignore 1 every second time. Looks like a bug
in Python, but the return value doesn't seem to be documented
anywhere, and -1 works as expected... so let's just use that.
systemd-python: update documentation for new systemd-journal group