Merge bitcoin/bitcoin#24470: Disallow more unsafe string->path conversions allowed by path append operators

f64aa9c411 Disallow more unsafe string->path conversions allowed by path append operators (Ryan Ofsky)

Pull request description:

  Add more `fs::path` `operator/` and `operator+` overloads to prevent unsafe string->path conversions on Windows that would cause strings to be decoded according to the current Windows locale & code page instead of the correct string encoding.

  Update application code to deal with loss of implicit string->path conversions by calling `fs::u8path` or `fs::PathFromString` explicitly, or by just changing variable types from `std::string` to `fs::path` to avoid conversions altogether, or make them happen earlier.

  In all cases, there's no change in behavior either (1) because strings only contained ASCII characters and would be decoded the same regardless of what encoding was used, or (2) because of the 1:1 mapping between paths and strings using the `PathToString` and `PathFromString` functions.

  Motivation for this PR was just that I was experimenting with #24469 and noticed that operations like `fs::path / std::string` were allowed, and I thought it would be better not to allow them.

ACKs for top commit:
  hebasto:
    ACK f64aa9c411

Tree-SHA512: 944cce49ed51537ee7a35ea4ea7f5feaf0c8fff2fa67ee81ec5adebfd3dcbaf41b73eb35e49973d5f852620367f13506fd12a7a9b5ae3a7a0007414d5c9df50f

src/addrdb.cpp

@@ -54,7 +54,7 @@ bool SerializeFileDB(const std::string& prefix, const fs::path& path, const Data
     std::string tmpfn = strprintf("%s.%04x", prefix, randv);
 
     // open temp output file, and associate with CAutoFile
-    fs::path pathTmp = gArgs.GetDataDirNet() / tmpfn;
+    fs::path pathTmp = gArgs.GetDataDirNet() / fs::u8path(tmpfn);
     FILE *file = fsbridge::fopen(pathTmp, "wb");
     CAutoFile fileout(file, SER_DISK, version);
     if (fileout.IsNull()) {