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

configure.ac

@@ -1064,7 +1064,24 @@ if $glib_failed ; then
 fi
 
 AC_CHECK_FUNCS(getprotobyname_r endservent if_nametoindex)
-AC_CHECK_HEADERS([netdb.h wspiapi.h arpa/nameser_compat.h])
+
+# <wspiapi.h> in the Windows SDK and in mingw-w64 has wrappers for
+# inline workarounds for getaddrinfo, getnameinfo and freeaddrinfo if
+# they aren't present at run-time (on Windows 2000).
+AC_CHECK_HEADER([wspiapi.h], [WSPIAPI_INCLUDE="\#include <wspiapi.h>"])
+AC_SUBST(WSPIAPI_INCLUDE)
+
+AC_MSG_CHECKING([if arpa/nameser_compat.h is needed])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([#include <arpa/nameser.h>],
+                                   [int qclass = C_IN;])],
+                  [AC_MSG_RESULT([no])],
+                  [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([#include <arpa/nameser.h>
+                                                       #include <arpa/nameser_compat.h>],
+                                                      [int qclass = C_IN;])],
+                                     [AC_MSG_RESULT([yes])
+                                      NAMESER_COMPAT_INCLUDE="\#include <arpa/nameser_compat.h>"],
+                                     [AC_MSG_ERROR([could not compile test program either way])])])
+AC_SUBST(NAMESER_COMPAT_INCLUDE)
 
 AS_IF([test $glib_native_win32 = no], [
   # We can't just use AC_CHECK_FUNC/AC_CHECK_LIB here. Bug 586150
@@ -3618,6 +3635,7 @@ gthread/Makefile
 gio/Makefile
 gio/gdbus-2.0/codegen/Makefile
 gio/gdbus-2.0/codegen/config.py
+gio/gnetworking.h
 gio/xdgmime/Makefile
 gio/inotify/Makefile
 gio/kqueue/Makefile