The idea is run psrf as a daemon, which is woken when somebody runs
the psrf equivalent of 'crontab -' to rewrite their psrf tab or
when a job (by *any* of the users with a psrf tab) is due to be
run. At that time the psrf daemon would put the current time (or
better the time at which it was *due* to run) in the enviroment
and fork a wrapper to actually su to the user and run the job.

All of this would rely on pysigset. So here's my notes on that:

farfalle$ mkdir -p ~/.venvs
farfalle$ python3 -m venv ~/.venvs/pysigset
farfalle$ ~/.venvs/pysigset/bin/python -m pip install wheel
Collecting wheel
  Using cached wheel-0.45.1-py3-none-any.whl (72 kB)
Installing collected packages: wheel
Successfully installed wheel-0.45.1
farfalle$ ~/.venvs/pysigset/bin/python -m pip install pysigset
Collecting pysigset
  Using cached pysigset-0.4.0.tar.gz (4.7 kB)
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: pysigset
  Building wheel for pysigset (setup.py) ... done
  Created wheel for pysigset: filename=pysigset-0.4.0-py3-none-any.whl size=5169 sha256=e22d4087244e0bd7628387ba735531dde8704ea728f016b1edf2beb5f5164098
  Stored in directory: /home/alexis/.cache/pip/wheels/cc/44/74/2942fd448b32d5f4bf6e231d3cebde3726033a32bbed269c1f
Successfully built pysigset
Installing collected packages: pysigset
Successfully installed pysigset-0.4.0
farfalle$