Update vendored/nonstd to latest and fix API usage

DEPENDENCIES

@@ -9,6 +9,7 @@
 * scope_guard: https://github.com/Neargye/scope_guard
 * subprocess: https://github.com/arun11299/cpp-subprocess
 * CLI11: https://github.com/CLIUtils/CLI11
+* string-lite: https://github.com/martinmoene/string-lite
 * boost
   * download boost
   * ./bootstrap.sh

src/config.cpp

@@ -512,7 +512,7 @@ Config::is_mergerfs_xattr(const char *attrname_)
 bool
 Config::is_cmd_xattr(const std::string_view &attrname_)
 {
-  return nonstd::string::starts_with(attrname_,"user.mergerfs.cmd.");
+  return nonstd::starts_with(attrname_,"user.mergerfs.cmd.");
 }
 
 std::string