logind: close race on session state during logins

At login there is a small race window where session_get_state() will
return SESSION_ACTIVE instead of SESSION_OPENING. This must be fixed
since during that time there are calls to session_save() to save
session states and we want to write the correct state.

When we queue the start scope and service jobs, we wait for both of them
to finish before calling and continue processing in:
"session_jobs_reply() => session_send_create_reply()"
to create the session fifo and notify clients.

However, in the match_job_removed() D-Bus signal, we may hit situations
where the scope job has successfully finished and we are still waiting
for the user service job to finish. During that time the
"session->scope_job" will be freed and set to NULL, this makes
session_get_state() return SESSION_ACTIVE before it is really active, it
should return SESSION_OPENING since we are still waiting for the service
job to finish in order to create the session fifo.

To fix this, we also check if the session fifo fd was created, if so then
the session has entered the SESSION_ACTIVE state, if not then it is still
in the SESSION_OPENING state and it is waiting for the scope and service
jobs to finish.
Extract looping over /proc/cmdline into a shared function

In cryptsetup-generator automatic cleanup had to be replaced
with manual cleanup, and the code gets a bit longer. But existing
code had the issue that it returned negative values from main(),
which was wrong, so should be reworked anyway.
Some modernizations
Pass log config from systemd to systemd-shutdown

If PID 1 debug logging is enabled, it is nice to keep those settings
when switching to systemd-shutdown binary, independently of whether
this was done through /proc/cmdline options, or through runtime
manipulations.