wpa_supplicant: systemd service directives

[holomorph]

Change the Requires= directive to BindsTo=.  This will cause
wpa_supplicant -i<iface> to terminate if <iface> disappears.

From 399ec170: Use correct (multi-user) target when installing systemd
units [...] all services should always be installed in multi-user.target

These directives are documented in systemd.unit(5)

[Earnestly]

This would help obviate a lot of issues guiding new users relating to fixing the systemd unit files.

[jmalinen]

Could you please read the top level CONTRIBUTIONS file and include Signed-hostap: tag in the commit message as described in that file?

[jmalinen]

Please also describe why it is desirable for wpa_supplicant to exit if the interface disappears. That case can happen, e.g., with Wi-Fi USB dongles and wpa_supplicant detects when such a device is re-inserted and then continues with the existing configuration. If that is forced to exit on interface removal, this use case would be broken.

[holomorph]

I was not aware of that use case. I just tried what you described with a bonding interface and I agree with you. I will amend the commit.

[holomorph]

Thoughts?

[Earnestly]

Please merge this, the current units are broken.

[boucman]

This is still broken, and actuallly poses real problems now...

systemd upstream recommands using preset-all for enabling units which will create wrong links. there is an explicit check to avoid that in the WantedBy path