Added HELP_MEDIA_NODUP and YELP_INIT_HELP

HELP_MEDIA normally craetes symlinks for localized copies that don't
exist, but for images this isn't actually necessary. Yelp can load
according to the document path. Don't want to break API on HELP_MEDIA
because people use it for a few other things. Hence, HELP_MEDIA_NODUP
which works just like HELP_MEDIA, except no symlinks on install.

Also, if you use HELP_MEDIA_NODUP and somebody builds with an older
yelp.m4, the files are just silently ignored. Added YELP_INIT_HELP
to replace YELP_HELP_INIT (though YELP_HELP_INIT is not currently
deprecated), and YELP_INIT_HELP is stingy about checking minimum
version.