Add stpcpy().

Like strcpy() but returning pointer to ending zero of copied string.

libsrc/common/stpcpy.c

@@ -0,0 +1,8 @@
+#include <string.h>
+
+char * __fastcall__
+stpcpy (char * dst, const char * src)
+{
+    strcpy (dst, src);
+    return dst + strlen (src);
+}