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
GDatagramBased is an interface abstracting datagram-based communications
in the style of the Berkeley sockets API. It may be contrasted to (for
example) GIOStream, which supports only streaming I/O.
GDatagramBased allows socket-like communications to be done through any
object, not just a concrete GSocket (which wraps socket()).
This adds the GDatagramBased interface, and implements it in GSocket.
https://bugzilla.gnome.org/show_bug.cgi?id=697907
