Sub-directories inside gio/ already processed in a previous commit:
- fam/
- gdbus-2.0/ (which contains only codegen/)
- gvdb/
- inotify/
- tests/
- win32/
- xdgmime/

Other sub-directories inside gio/:
- completion/: no license headers
- kqueue/: not LGPL, BSD-style license

https://bugzilla.gnome.org/show_bug.cgi?id=776504

If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.

It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.

https://bugzilla.gnome.org/show_bug.cgi?id=730493

Instead of "Return value:".

gtk-doc doesn't need <!-- --> anymore to recognize plural
forms of links.

Move g_pollable_source_new() here from gpollableinputstream.c, add
g_pollable_source_new_full(), and add some new methods to do either
blocking or nonblocking reads depending on a boolean argument.

https://bugzilla.gnome.org/show_bug.cgi?id=673997

Make g_pollable_input_stream_read() and
g_pollable_output_stream_write() look a little bit more like the
non-pollable versions in terms of error handling, etc. Also, use the
read_fn and write_fn virtual methods directly rather than calling
g_input_stream_read()/g_output_stream_write(), to avoid problems with
re-entrancy involving the "pending" flag.

Also belatedly add single-include guards to the header files.

https://bugzilla.gnome.org/show_bug.cgi?id=673997

GSource has been introspectable for a while, so can stop skipping
methods of GPollableInputStream/OutputStream that return it.

https://bugzilla.gnome.org/show_bug.cgi?id=664302