This is a set of changes to the way that URLs are handled
as paths. This is in support of eventual Zarr support.
The primary difference is to extract io handler information
from the url. This will be used inside dispatchers that support
multiple IO access methods (like file versus Zarr) to figure out
what internal IO handler to use.

Specific changes:
1. move url analyzer code from dfile.c to durlmodel.c
2. add code to walk the .daprc triples
3. NC_HDF5_finalize was not being properly called by nc_finalize().
4. Fix minor bug in ncgen3.l
5. fix memory leak in nc4info.c
6. replace protocol= fragment tag with a more general mode= tag.

This PR has two purposes:

1. Try to centralize all the code that infers the dispatcher
   from various available pieces of information in the path,
   the mode flags, and in the dataset itself.
2. Modify URL handling in support of using S3 URLS for S3
   reading and for Zarr. The goal is to make specific IO
   handling tags accessible inside dispatchers that support
   multiple IO access methods (like file versus Zarr) to figure
   out what internal IO handler to use.

The primary code change is to create a new set of files
libdispatch/dinfermodel.c and include/ncinfermodel.c
to centralize the dispatcher inference code.

Aside: the whole dispatcher inference mechanism needs serious fixing:
it is way too ad hoc.

Other changes:

1. NC_HDF5_finalize was not being properly called by nc_finalize().
2. Fix minor bug in ncgen3.l
3. fix memory leak in nc4info.c
4. add code to walk the .daprc triples and to replace protocol=
   fragment tag with a more general mode= tag.