Moves the following wallet load functions to a new wallet/load unit in
the libbitcoin_wallet library. All other functions in wallet/init remain
in libbitcoin_server:
- `VerifyWallets`
- `LoadWallets`
- `StartWallets`
- `FlushWallets`
- `StopWallets`
- `UnloadWallets`
- Add available WIN PID function
- Consider WIN32 in each relevant case
- Add new preprocessor definitions to suppress warning
- Update error message for generic OS
Co-authored-by: Jörn Röder <kontakt@joernroeder.de>
