Made find() return -1 if not found.

testing/zstring_test.cpp

@@ -94,5 +94,10 @@ int main(int argc, char **argv) {
 
    coreutils::ZString test17("   hello there   ");
    std::cout << "[" << test17.trim() << "]" << std::endl;
+
+   coreutils::ZString test18("this.test");
+   std::cout << "[" << test18.find(".") << "] " << test18.parsed() << ":" << test18.unparsed() << std::endl;
+   test18.reset();
+   std::cout << "[" << test18.find("x") << "] " << test18.parsed() << ":" << test18.unparsed() << std::endl;
    
 }