RFE: sd_pid_get_unit and related functions from sd-login

[jbosboom]

sd_pid_get_unit and related functions provided by sd-login do not use D-Bus and thus seem appropriate for this library. They are "just" string manipulation based on /proc/, but they don't seem trivial enough to just reimplement.

I haven't worked with the Python C API before, but if the maintainers agree this is in scope, I can take a stab at implementing them.