Dicache work

Hi there,

  There're  three  new  dicache  patches  in the repo. These have
beeen tested/checked with valgrind and gcc's address sanitizer.

  Given  the  last  memory  usage  reports,  and a very necessary
cleanup that was pending for a long time, this patch set tries to
tackle the issue.

  There's  also some decompressed image caching enabled (read the
verbose changelog) that will make dillo feel faster.

  Please test and send feedback.

  BTW, I found ways to crash dillo without the patches and wonder
whether some of you also experimented image-related crashes.

-- 
  Cheers
  Jorge.-