Log to file by default and allow specifying log file location

Log to file by default, matching Bitcoin Core.

Also allow passing a log file location as an argument:

```
-debuglogfile=<file>
    Specify location of debug log file (default: debug.log). Relative paths
    will be prefixed by a net-specific datadir location. Pass
    -nodebuglogfile to disable writing the log to a file.
```