* mh-compat.el (mh-image-load-path-for-library): Incorporate changes

* mh-compat.el (mh-image-load-path-for-library): Incorporate changes
from image-load-path-for-library, which are:
(image-load-path-for-library): Pass value of path rather than symbol.
Always return list of directories. Guarantee that image directory
comes first.

* mh-e.el (image-load-path): Define on those Emacsen that lack it to
avoid compile and run-time errors.

* mh-folder.el (mh-folder-mode): Use new idiom for setting
image-load-path.

* mh-letter.el (mh-letter-mode): Ditto.

* mh-utils.el (mh-logo-display): Ditto.