Implement qstrncpy() in terms of std::strncat()

This has the advantage that we're only copying strlen(src) characters,
like in the strncpy_s() case, not fill all of [dst,len) with NULs,
like strncpy() is wont to do.

[ChangeLog][Important Behavior Changes][qstrncpy()] On non-Windows
platforms, no longer writes to all bytes of the target buffer, but
stops after the terminating NUL. This was already the behavior on
Windows.

Change-Id: Ic86206f418affae6d0d88dfe79537eeb833a7daa
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qbytearray.cpp

@@ -133,16 +133,9 @@ char *qstrcpy(char *dst, const char *src)
 char *qstrncpy(char *dst, const char *src, size_t len)
 {
     if (dst && len > 0) {
-        if (!src) {
-            *dst = '\0';
-            return nullptr;
-        }
-#ifdef Q_CC_MSVC
-        strncpy_s(dst, len, src, len - 1);
-#else
-        strncpy(dst, src, len);
-#endif
-        dst[len-1] = '\0';
+        *dst = '\0';
+        if (src)
+            std::strncat(dst, src, len - 1);
     }
     return src ? dst : nullptr;
 }