bootstrap: consistently use correct copyright year

bootstrap: consistently use correct copyright year

Suggested by Peter Johansson in the discussion about automake bug#11356:
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11356#41>

* boostrap.sh (dosubst): Don't initialize the substituted value for the
release year from the current year (using `date +%Y`); instead, do it
through ...
($RELEASE_YEAR): ... this new statically-defined variable.  To ensure it
won't get out-of-sync, it will be automatically updated ...
* Makefile.am (update-copyright): ... by this target's recipe.
Since are at it, fix a botched output redirection for an error message,
i.e., use ">&2" instead of ">&1".

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>