Transition to KDF.json from MM2.json

[gcharang]

We changed the name of the binary from mm2 to kdf. It might be appropriate to change the name of the default config file too.

We should also similarly change the names of other files etc that are publicly seen. Example: MM2.db

[onur-ozkan]

We should likely add a mechanism to rename existing MM2 files when possible to avoid breaking changes.

It may not be posible due to user permissions, in that case it should fallback to using MM2 file.