A few tweaks to RefBase debugging

- stacks are now saved in /data/debug which must be
  created and writable by the user.

- removed "always fatal" DEBUG_REFS option, it wasn't
  really needed.

- DEBUG_REFS_ENABLED_BY_DEFAULT is not the default anymore
  (usually people want to target which refs they're tracking)

Change-Id: I37fae72e9dacde6ce1fa8f7dbe2bc01b1a1b95e5