* gio/giomm/socketsource.cc: Add extern "C".
* gio/src/application.[ccg|hg]: Call Application_Class::open_callback()
via a local function with C linkage.
* gio/src/asyncinitable.[ccg|hg]: Call
AsyncInitable_Class::init_async_vfunc_callback() and
init_finish_vfunc_callback() via local functions with C linkage.
* gio/src/dbusobjectmanagerclient.ccg: Use a local function with C linkage
instead of Glib::destroy_notify_delete<SlotProxyType>.
* gio/src/file.ccg: Add extern "C".
* gio/src/liststore.ccg: Add a TODO comment.
* gio/src/memoryinputstream.ccg: Add extern "C".
* gio/src/settings.ccg: Add extern "C".
* gio/src/socketcontrolmessage.[ccg|hg]: Call
SocketControlMessage_Class::deserialize_async_vfunc_callback()
via a local function with C linkage.

Code that mixes up C linkage and C++ linkage has undefined behavior.
Most compilers make no difference between C and C++ linkage, so it
has not been an issue so far. Part of issue #1