move kill mechanism to public API

the vips_image_set_kill() system was internal. Move it to the public
API.

See eg.:

https://github.com/kleisauke/net-vips/issues/31

ChangeLog

@@ -29,6 +29,7 @@
   processes [jtorresfabra]
 - add vips_region_fetch() / _width() / _height() for language bindings
 - vips_text() supports justification
+- move vips_image_set_kill() and iskilled() to the public API
 
 4/1/19 started 8.7.4
 - magickload with magick6 API did not chain exceptions correctly causing a

libvips/include/vips/image.h

@@ -427,6 +427,8 @@ void vips_image_invalidate_all( VipsImage *image );
 void vips_image_minimise_all( VipsImage *image );
 
 void vips_image_set_progress( VipsImage *image, gboolean progress );
+gboolean vips_image_iskilled( VipsImage *image );
+void vips_image_set_kill( VipsImage *image, gboolean kill );
 
 char *vips_filename_get_filename( const char *vips_filename );
 char *vips_filename_get_options( const char *vips_filename );

libvips/include/vips/internal.h

@@ -153,8 +153,6 @@ int vips_image_written( VipsImage *image );
 void vips_image_preeval( VipsImage *image );
 void vips_image_eval( VipsImage *image, guint64 processed );
 void vips_image_posteval( VipsImage *image );
-gboolean vips_image_iskilled( VipsImage *image );
-void vips_image_set_kill( VipsImage *image, gboolean kill );
 VipsImage *vips_image_new_mode( const char *filename, const char *mode );
 
 int vips__formatalike_vec( VipsImage **in, VipsImage **out, int n );

libvips/iofuncs/image.c

@@ -1598,19 +1598,18 @@ vips_image_set_progress( VipsImage *image, gboolean progress )
 		image->progress_signal = NULL;
 }
 
-
 /**
  * vips_image_iskilled: (method)
  * @image: image to test
  *
  * If @image has been killed (see vips_image_set_kill()), set an error message,
- * clear the #VipsImage.kill flag and return %FALSE. Otherwise return %TRUE.
+ * clear the #VipsImage.kill flag and return %TRUE. Otherwise return %FALSE.
  *
  * Handy for loops which need to run sets of threads which can fail. 
  *
  * See also: vips_image_set_kill().
  *
- * Returns: %FALSE if @image has been killed. 
+ * Returns: %TRUE if @image has been killed. 
  */
 gboolean
 vips_image_iskilled( VipsImage *image )
@@ -1645,8 +1644,6 @@ vips_image_iskilled( VipsImage *image )
  * threads. 
  *
  * See also: vips_image_iskilled().
- *
- * Returns: %FALSE if @image has been killed. 
  */
 void
 vips_image_set_kill( VipsImage *image, gboolean kill )