camtool: document reindex-diskpacked better

Change-Id: Ib2f669ddf4fa493572a811b0564f104fbaeea05a
Merge branch 'master' of https://camlistore.googlesource.com/camlistore
Diskpacked: support deletion

Delete from index and zero out blob's data.
Use FALLOC_FL_PUNCH_HOLE if available (linux >= 2.6.38).

Change-Id: I0c194fd6734a35b520212398d93dfdc5cabadeb5