contrib/init: Update openrc-run filename

OpenRC changed their program binary names in 2014 (3 years ago), and using the old names has loud warnings now

Github-Pull: #11676
Rebased-From: 2f041f0e7d

contrib/init/bitcoind.openrc

@@ -1,4 +1,4 @@
-#!/sbin/runscript
+#!/sbin/openrc-run
 
 # backward compatibility for existing gentoo layout 
 #