Do much more consistency checking when storing and resuming the clock.

Do much more consistency checking when storing and resuming the clock.

Saving the clock history and the running clock could lead to pointers
to non-existing buffer, or to dired buffers of directories.  This
patch implements more consistency checks.

It also allows a new value for `org-clock-persist'.  When this is
`clock', only the running clock, but not the entire clock history
is saved.