Creates nested <network> directory instead of using existing one

This is my first time playing with `bmultisig` so I could be using it wrong...
Seeing unexpected behavior in its data directory creation.

I'm running it with `--prefix= .../bcoin/regtest` and it is correctly finding the `wallet.conf` file in that location. (ports and API keys are parsed correctly and it seems to be running fine)
but then, unlike bwallet, it creates a NEW directory `regtest` INSIDE its prefix directory:


Reproduce the behavior using bmultisig with --prefix set to a bcoin/regtest directory containing wallet.conf, then trace the data-directory setup and compare it with bwallet's handling of the same prefix. Done means bmultisig uses the existing prefix without creating a nested regtest directory and still reads wallet.conf correctly.

Written by the indexing model from the issue text.