fixed bug in the pagecache which lead to assertion in multithread test


include/pagecache.h:
  post review fixes:
   - comments fixed
   - types fixed
   - keyword 'extern' added to all interface functions
mysys/mf_pagecache.c:
  postreview fixes:
   - comments fixed and added
   - types fixed
   - typo fixed
  Added write locking flag set when we took a block from LRU and going to free it (to prevent locking/using it for write)
mysys/test_pagecache_consist.c:
  pagecache size reduced (to be able reproduce problems found by Guilhem)
  typo in the comment fixed