Datadir option in linearize scripts

Adds a datadir configuration option to the linearize scripts to allow the script to use the RPC cookie instead of requiring the user to set a rpcuser and rpcpassword for the rpc server.

contrib/linearize/example-linearize.cfg

@@ -1,6 +1,7 @@
 # bitcoind RPC settings (linearize-hashes)
 rpcuser=someuser
 rpcpassword=somepassword
+#datadir=~/.bitcoin
 host=127.0.0.1
 port=8332
 #port=18332