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/README.md

@@ -7,7 +7,8 @@ run using Python 3 but are compatible with Python 2.
     $ ./linearize-hashes.py linearize.cfg > hashlist.txt
 
 Required configuration file settings for linearize-hashes:
-* RPC: `rpcuser`, `rpcpassword`
+* RPC: `datadir` (Required if `rpcuser` and `rpcpassword` are not specified)
+* RPC: `rpcuser`, `rpcpassword` (Required if `datadir` is not specified)
 
 Optional config file setting for linearize-hashes:
 * RPC: `host`  (Default: `127.0.0.1`)