sd-network: ignore missing directory if networkd is disabled

[michaelolbrich]

sd_network_monitor is used by timesyncd and resolved. Both can be enabled
while networkd is disabled. /run/systemd/netif/links/ only exists if
networkd is enabled at build-time, so ignore the corresponding error if it
is disabled.
The user that owns /run/systemd/netif/links/ may not exist if networkd is
disabled, so always creating this directory is not possible.

[poettering]

Hmm, I think i'd relly prefer a more complex patch:

if we cannot add /run/systemd/netif/links/ to the inotify watch, then add /run/systemd/netif instead looking for new subdirs being created for that, If we cannot add this either, then add /run/systemd instead, looking for new subdirs.

Then, in sd_network_monitor_flush() actually iterate through the inotify events using the FOREACH_INOTIFY_EVENT() (which makes this very easy), and when we get events for /run/systemd or /run/systemd/netif try to readd /run/systemd/netif, and drop the other watches if that worked.

That way we can run networkd and its clients completely racefreely in parallel without any specific ordering or reduction in functionality.

[poettering]

Hmm, sorry for closing the PR. That only means i think the patch needs more love, i do believe the issue should be fixed in general.

[poettering]

github workflow is really weird, sorry.

[poettering]

I created a new issue to track this as #107. Please make sure to reference that when posting a new PR.