Add gnetworking.h

Install a public "gnetworking.h" header that can be used to include
the relevant OS-dependent networking headers. This does not really
abstract away unix-vs-windows however; error codes, in particular,
are incompatible.

gnetworkingprivate.h now contains just a few internal URI-related
functions

Also add a g_networking_init() function to gnetworking.h, which can be
used to explicitly initialize OS-level networking, rather than having
that happen as a side-effect of registering GInetAddress.

https://bugzilla.gnome.org/show_bug.cgi?id=623187

docs/reference/gio/gio-docs.xml

@@ -122,6 +122,7 @@
       <xi:include href="xml/gunixcredentialsmessage.xml"/>
       <xi:include href="xml/gproxy.xml"/>
       <xi:include href="xml/gproxyaddress.xml"/>
+      <xi:include href="xml/gnetworking.xml"/>
     </chapter>
     <chapter id="highlevel-socket">
       <title>High-level network functionallity</title>

docs/reference/gio/gio-sections.txt

@@ -3981,3 +3981,9 @@ G_IS_TASK_CLASS
 G_TASK_GET_CLASS
 g_task_get_type
 </SECTION>
+
+<SECTION>
+<FILE>gnetworking</FILE>
+<TITLE>gnetworking.h</TITLE>
+g_networking_init
+</SECTION>