addpm.c: Replace existing entries, but do not create new ones

addpm.c: Replace existing entries, but do not create new ones

* nt/addpm.c (add_registry): If the Emacs registry key exists, replace
existing values from previous versions, but do not add new ones; the
key could exist for other reasons unrelated to old Emacsen, like X-style
resources, or to set some environment variables like HOME or LANG, and
in that case we don't want to populate it with obsolete values.