glib-unix: New Unix-specific API

GLib historically has been designed to be "mostly" portable; there
are some functions only available on Unix like g_io_channel_unix_new(),
but these are typically paired with obvious counterparts for Win32.

However, as GLib is used not only by portable software, but components
targeting Unix (or even just Linux), there are a few cases where it
would be very convenient if GLib shipped built-in functionality.

This initial patch is a basic wrapper around pipe2(), including
fallbacks for older kernels.  This pairs well with the
existing g_spawn_*() API and its child_setup functionality.

However, in the future, I want to add a signal() wrapper here,
complete with proxying the signal to a mainloop.  I have initial code
for this, but doing it sanely (including factoring out gmain.c's
private worker thread), is a complex task, and I don't want to block
on that.

See also gwin32.h for Win32 specific functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=644941

docs/reference/glib/glib-sections.txt

@@ -1924,6 +1924,14 @@ g_win32_ftruncate
 
 </SECTION>
 
+<SECTION>
+<TITLE>UNIX Compatibility Functions</TITLE>
+<FILE>gunix</FILE>
+G_UNIX_ERROR
+g_unix_pipe_flags
+
+</SECTION>
+
 # Data Structures
 
 <SECTION>