Merge bitcoin#17085: init: Change fallback locale to C.UTF-8

facb9a1 init: Change fallback locale to C.UTF-8 (Wladimir J. van der Laan)

Pull request description:

  Much of our code assumes file system 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`.

  related: bitcoin#14948 (comment)

ACKs for top commit:
  MarcoFalke:
    ACK facb9a1

Tree-SHA512: 5075f9fe6791572d76ec38c58cd56f04ed8086c06a7d7f446d062dffc313c62466ba81f1a7d6b8c7e95791fcff82e4f76871c3534478fbfe5beb456dd8eea340

Author: laanwj

src/util/system.cpp

@@ -1266,12 +1266,12 @@ 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);
     }
 #endif
     // The path locale is lazy initialized and to avoid deinitialization errors