netcdf-c

Primary change is to add ability
to do prefetch on either a lazy
or eager basis. Lazy means that
the prefetch does not occur
until and unless the client actually
makes a get_var request.

Also repaired a problem where
doing prefetch wrt a url that
has a constraint will prefetch
a whole variable if its constrained
size is small enough, even if the
underlying variable is too large
to warrant prefetch.