refactor: Block unsafe std::string fs::path conversion copy_file calls

There is no change in behavior. This just helps prepare for the
transition from boost::filesystem to std::filesystem by avoiding
copy_file calls that will be unsafe after the transition to
std::filesystem to due lack of a boost::filesystem::path::imbue
equivalent and inability to set a predictable locale.

Author: hebasto

src/fs.h

@@ -92,6 +92,13 @@ static inline path operator+(path p1, path p2)
     return p1;
 }
 
+// Disallow implicit std::string conversion for copy_file
+// to avoid locale-dependent encoding on Windows.
+static inline void copy_file(const path& from, const path& to, copy_option options)
+{
+    boost::filesystem::copy_file(from, to, options);
+}
+
 /**
  * Convert path object to byte string. On POSIX, paths natively are byte
  * strings, so this is trivial. On Windows, paths natively are Unicode, so an

src/wallet/wallet.cpp

@@ -377,7 +377,7 @@ std::shared_ptr<CWallet> RestoreWallet(WalletContext& context, const std::string
     }
 
     auto wallet_file = wallet_path / "wallet.dat";
-    fs::copy_file(backup_file, wallet_file, fs::copy_option::fail_if_exists);
+    fs::copy_file(fs::u8path(backup_file), wallet_file, fs::copy_option::fail_if_exists);
 
     auto wallet = LoadWallet(context, wallet_name, load_on_start, options, status, error, warnings);