Change the default datadir to bitcoincash, not bitcoin.

### Describe the issue

### Can you reliably reproduce the issue?
#### If so, please list the steps to reproduce below:
1. start bitcoin cash

### Expected behaviour
Should create a directory, `~/.bitcoincash`

### Actual behaviour
It uses `~/.bitcoin` and writes to bitcoins debug.log, and other files.

### What version of bitcoin-abc are you using?
2a10748d48f9a895f349b6b8411461acff777dc5 compiled from source

### Machine specs:
- OS: Ubuntu
- CPU: na
- RAM: na
- Disk size: na
- Disk Type (HD/SDD): na