rpc: Generate auth cookie in hex instead of base64
rpc: Generate auth cookie in hex instead of base64

>>> backports bitcoin@1c80386bceb216ca5b5da657e03a29f9c779d58b

Base64 contains '/', and the '/' character in credentials is problematic
for AuthServiceProxy which represents the RPC endpoint as an URI with
user and password embedded.
Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX