- Closes#366 and closes#304 (all sub-tasks).
- Start new process group with setsid() by sending
start_new_session=True
- Detach from parent process fds by closing all FDs and attaching stdin,
stdou and stderr to /dev/null.
- Don't wait for the process to complete.
- This allows for upgrading Plinth while upgrades are trigged from
Plinth itself.
- Show log of upgrade exection instead of output and error log of the
process which can no longer be collected. This has the advantage of
showing automatic executions also.
- Rewrite the mechanism to detect whether upgrades can be run. It is
now based on whether the package manager is busy. This has the
advantage of working properly if other apt processes are running,
automatic upgrades are running, etc.
- Busy status works even if Plinth is restarted while upgrades are in
progress.
- More descriptive messages showing that upgrades don't have to be
triggered manually.
- Warn that other packages can't be installed while upgrades are
running, which may take a long time.
- Warn the users of potential temporary unavailability of
Plinth/Apache2.
- Show error message based on return code rather than messages in
stderr.
- Don't decorate the message paragraph with alert color, we are already
doing that by showing a message at the top.
- Untabify.
- Improve message showing that upgrades are running, gramatically.
- Show errors messages decorated as errors.
- Minor cleanups.
