util: extract {Read,Write}BinaryFile() to its own files

Extract `ReadBinaryFile()` and `WriteBinaryFile()` from `torcontrol.cpp`
to its own `readwritefile.{h,cpp}` files, so that it can be reused from
other modules.

src/util/readwritefile.cpp

@@ -0,0 +1,47 @@
+// Copyright (c) 2015-2020 The Bitcoin Core developers
+// Copyright (c) 2017 The Zcash developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <fs.h>
+
+#include <limits>
+#include <stdio.h>
+#include <string>
+#include <utility>
+
+std::pair<bool,std::string> ReadBinaryFile(const fs::path &filename, size_t maxsize=std::numeric_limits<size_t>::max())
+{
+    FILE *f = fsbridge::fopen(filename, "rb");
+    if (f == nullptr)
+        return std::make_pair(false,"");
+    std::string retval;
+    char buffer[128];
+    size_t n;
+    while ((n=fread(buffer, 1, sizeof(buffer), f)) > 0) {
+        // Check for reading errors so we don't return any data if we couldn't
+        // read the entire file (or up to maxsize)
+        if (ferror(f)) {
+            fclose(f);
+            return std::make_pair(false,"");
+        }
+        retval.append(buffer, buffer+n);
+        if (retval.size() > maxsize)
+            break;
+    }
+    fclose(f);
+    return std::make_pair(true,retval);
+}
+
+bool WriteBinaryFile(const fs::path &filename, const std::string &data)
+{
+    FILE *f = fsbridge::fopen(filename, "wb");
+    if (f == nullptr)
+        return false;
+    if (fwrite(data.data(), 1, data.size(), f) != data.size()) {
+        fclose(f);
+        return false;
+    }
+    fclose(f);
+    return true;
+}