SELinux interface support

[mhall119]

@Conan_Kudo

Migrating most resource access to Portals is not necessarily a bad move anyway, as it further generalizes the sandboxing/confinement mechanisms and reduces the reliance on MAC. However, back then, it was mostly thrown out as the idea of reusing Portals wasn’t appealing. I don’t know if that has changed now.

That work is getting started for the desktop portals like file access. But that will only solve a small part of the use-cases that Snapd interfaces cover.