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.
Backport of Core PR 10104
Test Plan: N/A as we do not currently use these scripts. Only reason we're backporting these changes is because they block backports that we do need.
Reviewers: deadalnix, schancel, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D2150