storage/maria/ha_maria.cc:
  enable LOAD INDEX for Maria
storage/maria/ma_preload.c:
  Making index preloading work for Maria.
storage/maria/maria_def.h:
  We don't need this #define anymore, ma_test_recovery.pl uses
  zerofill to do comparisons of physical files.
mysql-test/r/maria-preload.result:
  result. The values of maria_pagecache_reads look too high, see
  BUG#35030 "maria_pagecache_read* status variables updated at wrong moment"
mysql-test/t/maria-preload.test:
  Test of index preloading in Maria