the latter wrote into the settings of the key cache! Problem was that
--pagecache* options reused OPT_KEY_CACHE* enum values.


sql/mysqld.cc:
  --pagecache* options should not reuse the OPT_KEY_CACHE* enum values, it
  confuses my_getopt. Making handling of page caches similar to handling
  of key caches, in mysqld.cc.