init: Change fallback locale to C.UTF-8

Much of our code assumes UTF-8 support, and this is a more realistic
guess for modern systems anyway than the default character set (which
would be ASCII only). So change the assumed fallback locale (if no
locale is defined by the user or OS) to `C.UTF-8`.

Github-Pull: bitcoin#17085
Rebased-From: facb9a1

Author: laanwj

src/util/system.cpp

@@ -1153,12 +1153,12 @@ void SetupEnvironment()
     }
 #endif
     // On most POSIX systems (e.g. Linux, but not BSD) the environment's locale
-    // may be invalid, in which case the "C" locale is used as fallback.
+    // may be invalid, in which case the "C.UTF-8" locale is used as fallback.
 #if !defined(WIN32) && !defined(MAC_OSX) && !defined(__FreeBSD__) && !defined(__OpenBSD__)
     try {
         std::locale(""); // Raises a runtime error if current locale is invalid
     } catch (const std::runtime_error&) {
-        setenv("LC_ALL", "C", 1);
+        setenv("LC_ALL", "C.UTF-8", 1);
     }
 #elif defined(WIN32)
     // Set the default input/output charset is utf-8