interfaces_gre.php: Interface flags on GRE interfaces are incorrectly configured, always enabled when saving in GUI
When creating or editing a GRE interface the interface at the OS level (e.g. gre0) has all flags by default (LINK0, LINK1, LINK2) despite the GUI settings showing them disabled. Checking the boxes and saving has no effect.
When booting, the flags appear to be processed correctly.
Issue seems to be present on 2.2.x and 2.3.
I have a fix, will push momentarily.
Correct the code saving values for GRE. The GRE configure backend code tests for isset, the way the values were copied before it would be set to 'false' which was still set, so the code always triggered when it shouldn't. When reading back from XML at boot time it's OK, so only the web interface is affected. Fixes #6010