Allow translations to reorder string substitutions

This is supported on Windows
(https://docs.microsoft.com/en-us/cpp/c-runtime-library/printf-p-positional-parameters)
and on POSIX, so basically everywhere.

Add some tests to verify that the target system does indeed support
these positional parameters.

(cherry picked from commit ddd2b93190)

src/base/log.cpp

@@ -116,7 +116,7 @@ void log_log_impl(LEVEL level, bool have_color, LOG_COLOR color, const char *sys
 #pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif
 #if defined(CONF_FAMILY_WINDOWS)
-	_vsnprintf(pMessage, MessageSize, fmt, args);
+	_vsprintf_p(pMessage, MessageSize, fmt, args);
 #else
 	vsnprintf(pMessage, MessageSize, fmt, args);
 #endif

src/base/system.cpp

@@ -2657,7 +2657,7 @@ int str_format(char *buffer, int buffer_size, const char *format, ...)
 #if defined(CONF_FAMILY_WINDOWS)
 	va_list ap;
 	va_start(ap, format);
-	_vsnprintf(buffer, buffer_size, format, ap);
+	_vsprintf_p(buffer, buffer_size, format, ap);
 	va_end(ap);
 
 	buffer[buffer_size - 1] = 0; /* assure null termination */

src/test/str.cpp

@@ -217,6 +217,27 @@ TEST(Str, Endswith)
 		str_length(ABCDEFG) - str_length(DEFG));
 }
 
+TEST(StrFormat, Positional)
+{
+	char aBuf[256];
+
+	// normal
+	str_format(aBuf, sizeof(aBuf), "%s %s", "first", "second");
+	EXPECT_STREQ(aBuf, "first second");
+
+	// normal with positional arguments
+	str_format(aBuf, sizeof(aBuf), "%1$s %2$s", "first", "second");
+	EXPECT_STREQ(aBuf, "first second");
+
+	// reverse
+	str_format(aBuf, sizeof(aBuf), "%2$s %1$s", "first", "second");
+	EXPECT_STREQ(aBuf, "second first");
+
+	// duplicate
+	str_format(aBuf, sizeof(aBuf), "%1$s %1$s %2$d %1$s %2$d", "str", 1);
+	EXPECT_STREQ(aBuf, "str str 1 str 1");
+}
+
 TEST(Str, EndswithNocase)
 {
 	EXPECT_TRUE(str_endswith_nocase("abcdef", "deF"));