libsysprof: provide unwind pipe from client

We don't need a socketpair for this. Additionally, things seem to work
better from the service when the client provides the pipe. Otherwise, when
running as a dbus service I often have issues with things getting closed
out from under us.

src/sysprofd/org.gnome.Sysprof3.Unwinder.xml

@@ -17,7 +17,7 @@
       <arg type="u" name="stack_size" direction="in"/>
       <arg type="a(hi)" name="perf_fds" direction="in"/>
       <arg type="h" name="event_fd" direction="in"/>
-      <arg type="h" name="capture_fd" direction="out"/>
+      <arg type="h" name="capture_fd" direction="in"/>
     </method>
   </interface>
 </node>