Make add_to_log varargs

Make add_to_log varargs

* src/alloc.c (run_finalizer_handler):
* src/charset.c (load_charset_map_from_vector):
* src/nsimage.m (ns_load_image):
* src/xfaces.c (load_pixmap, load_color2):
Simplify, now that add_to_log has a variable number of args.
* src/image.c (image_error): Take a variable number of args.
Callers simplified.
* src/lisp.h (add_to_log, vadd_to_log): Adjust to new APIs.
* src/xdisp.c (format_nargs, vadd_to_log): New functions.
(add_to_log): Make varargs, and reimplement in terms of vadd_to_log.
* src/xfaces.c (merge_face_ref): Fix typo that omitted color name.