Reliable network connectivity for apt-daily

[Impact]

apt-daily.service is launched by a timer that depends on network-online.target (after the fixes for bug 1686470 are in everywhere)

At boot that is mostly sufficient for it to have network online, but it does not seem to work all the time, and we might be disagreeing with network-manager and friends what online state means.

At resume time, network-online.target is still active, so the service is started as soon as possible when it tries to catch up. Depending on the timing, the network connectivity might not be there yet, and it will fail and only retry 12 hours later.

[Proposed solution]
Introduce a new apt-helper wait-online that waits for the machine being online, using both network-manager and systemd-networkd helpers. If the service is active, we use the respective online wait helper to wait for it to signal onlineness. Once all helpers have reported onlineness, we continue.

[Original proposal, to be done later]
original plan:
It tries to connect() to remote hosts specified in sources.list until one connection works or a TIMEOUT is reached. The proposed algorithm looks something like this:

while (time elapsed < TIMEOUT):
  for each entry:
    host = getaddrinfo()
    if host failed:
      continue
    fd = connect to it
    if fd is invalid:
      continue

    all fds += fd

    if poll(all fds, 100 ms timeout) finds a connected one:
      exit(0)

exit(42) # timeout

There are two things to consider:
* getaddrinfo() and connect() may fail if network is not up yet, so we need to retry (we might need to sleep somewhere)
* If poll() fails, we likely sleep enough, so no extra sleep needed.

I believe the time out should be something like 30s.

On the systemd service side, we add:
  ExecStartPre=/usr/lib/apt/apt-helper wait-online
  RestartForceExitStatus=42
  RestartSec=15m

To retry the service after 15 minutes.

[Test case]
* Start apt-daily.service after turning off network -> It should wait (in ExecStartPre)
* Turn on network -> apt-daily.service should start

[Regression potential]
There might be increased I/O activity after resume, if that did not work before. The helper is launched in an ExecStartPre unit and failures are marked as ignored by "-". systemd automatically kills all ExecStartPre processes when the main ExecStartPre process exits, so there is no chance of ending with some child process still running.