Log Message:
Update to 4.1.21

Changelog:
Features
    --enable-memclean cleans up memory for use with memory checkers,
      eg. valgrind.
    refuse-any nsd.conf option that refuses queries of type ANY.
    lower memory usage for tcp connections, so tcp-count can be higher.

Bug Fixes
    Fix unused variable warnings and uninit variable in statistics printout
      from clang analyzer.
    Fix spelling error in xfr-inspect.
    Fix #3562: explain build error when flex missing.
    Fix buffer size warnings from compiler on filename lengths.
    Fix #4093: Release notes not using 2018.