Alpha & beta announcements shouldn't go to info-gnu by default

[Reuben Thomas]

Karl just pointed out to me that, as per:

http://www.gnu.org/prep/maintain/html_node/Announcements.html

non-stable release announcements should only go to info-gnu in
exceptional circumstances. Therefore, the hardwired To: address in
maint.mk is wrong, and should be set differently for alpha, beta &
stable targets.

I had a look through maint.mk, and it seems that the right way to
implement this is to pass RELEASE_TYPE down to the announcement
target, and then have To: default to info-gnu for stable releases, and
$(PACKAGE_BUGREPORT) otherwise, which in turn means that
announcement_Cc_ needs to add $(PACKAGE_BUGREPORT) only for stable
releases. Hence, the conditional assignments of announcement_Cc_ and
announcement_mail_headers_ need to be moved into the announcement
target, which seems harmless as those variables are not used elsewhere
in maint.mk.

In other words, I'm happy to provide a patch once the management has
approved the lines along which I'm writing it!

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