interfaces: misc small interface updates

[jdstrand]

• apparmor: allow read on @{PROC}/sys/fs/file-nr by default
• seccomp: better document ioctl mediation
• interfaces/desktop-legacy: allow org.gtk.GLib.PACRunner.Lookup()

[jdstrand]

@jhenstridge - can you comment on 65323a9 and its location in desktop-legacy?

[jhenstridge]

This seems fairly safe to add to desktop-legacy. We probably want to direct people towards org.freedesktop.portal.ProxyResolver instead though.

If a confined app is hitting this D-Bus API, then it is using the libgiognomeproxy GProxyResolver backend, and has gsettings access to know that proxy auto config has been enabled.

If instead portal support is enabled, the portal backend for GProxyResolver is used. This one doesn't need to have gsettings plugged, and delegates all proxy requests to xdg-desktop-portal, which in turn calls the standard GProxyResolver backend.

As far as security concerns over the org.gtk.GLib.PACRunner interface specifically, the client can ask the unconfined glib-pacrunner executable to execute an arbitrary file://, http://, or https:// URL as JavaScript and pass in an arbitrary URL as a function argument. With that said, the JS is run within mozjs's web sandbox so should be fairly safe.

[jdstrand]

@kenvandine - fyi, this came up due to a snap-store bug: https://bugs.launchpad.net/snapd/+bug/1871364 and based on @jhenstridge comments, it seems like it needs to be updated.