libsysprof-capture: add API to duplicate underlying write FD

This is useful for use in our other libraries that may need access to these
to create document loaders for a completed writer.

src/libsysprof-capture/sysprof-capture-writer.c

@@ -1704,3 +1704,14 @@ _sysprof_capture_writer_add_raw (SysprofCaptureWriter      *self,
 
   return true;
 }
+
+int
+_sysprof_capture_writer_dup_fd (SysprofCaptureWriter *self)
+{
+  assert (self != NULL);
+
+  if (self->fd == -1)
+    return -1;
+
+  return dup (self->fd);
+}

src/libsysprof-capture/sysprof-capture-writer.h

@@ -244,5 +244,7 @@ SYSPROF_INTERNAL
 bool                  _sysprof_capture_writer_set_time_range                 (SysprofCaptureWriter              *self,
                                                                               int64_t                            start_time,
                                                                               int64_t                            end_time) SYSPROF_INTERNAL;
+SYSPROF_INTERNAL
+int                   _sysprof_capture_writer_dup_fd                         (SysprofCaptureWriter              *self);
 
 SYSPROF_END_DECLS