Add SPDX license (but not copyright) headers to all files which follow a
certain pattern in their existing non-machine-readable header comment.
This commit was entirely generated using the command:
```
git ls-files gio/*.[ch] | xargs perl -0777 -pi -e 's/\n \*\n \* This library is free software; you can redistribute it and\/or\n \* modify it under the terms of the GNU Lesser General Public/\n \*\n \* SPDX-License-Identifier: LGPL-2.1-or-later\n \*\n \* This library is free software; you can redistribute it and\/or\n \* modify it under the terms of the GNU Lesser General Public/igs'
```
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Helps: #1415
These are private helper functions that will be used in
the following commits to get information about whether
we are running in a flatpak sandbox, etc.
We allow the use of GTK_USE_PORTAL=1 in the environment
to force the use of portals. This can be useful for
testing and debugging portal interaction.
https://bugzilla.gnome.org/show_bug.cgi?id=768498
