This ensures that `wallet_utils.py` and `genwallet.py` use the same logic for
reading the password.
The current algorithm for reading the password entails some
non-obvious details like utf-8 encoding and not stripping trailing
newlines.
Changes from top to bottom:
- Add header comment
- Import wallet_utils (needed for DEFAULT_MIXDEPTH)
- Make password optional in usage string
- Set variable wallet_name at the top where options are parsed
- Use DEFAULT_MIXDEPTH instead of hardcoded value
