This is by far the heaviest feature of rdo, justifying
its own file for its 140loc.
It creates sessions, inspired by the way doas does it.
We use the /run/rdo temporary folder to store files in the
format of /run/rdo/pid-ts, pid being the PID of the process
that executed rdo, and ts being the timestamp at which said
process started.
As no 2 processes can have the exact same PID and startup time
(startup time is measured in the milliseconds), this seems secure.
Closes#4.
