Replaces PR Unidata#3024

re: Unidata#2753

As suggested by Ed Hartnett, This PR extends the netcdf.h API to support programmatic control over the search path used to locate plugins.

I created several different APIs, but finally settled on the following API as being the simplest possible. It does have the disadvantage that it requires use of a global lock (not implemented) if used in a threaded environment.

Specifically, note that modifying the plugin path must be done "atomically". That is, in a multi-threaded environment, it is important that the sequence of actions involved in setting up the plugin path must be done by a single processor or in some other way as to guarantee that two or more processors are not simultaneously accessing the plugin path get/set operations.

As an example, assume there exists a mutex lock called PLUGINLOCK. Then any processor accessing the plugin paths should operate as follows:
````
lock(PLUGINLOCK);
nc_plugin_path_get(...);
<rebuild plugin path>
nc_plugin_path_set(...);
unlock(PLUGINLOCK);
````
## Internal Architecture

It is assumed here that there only needs to be a single set of plugin path directories that is shared by all filter code and is independent of any file descriptor; it is global in other words. This means, for example, that the path list for NCZarr and for HDF5 will always be the same.

However internally, processing the set of plugin paths depends on the particular NC_FORMATX value (NC_FORMATX_NC_HDF5 and NC_FORMATX_NCZARR, currently). So the *nc_plugin_path_set* function, will take the paths it is given and propagate them to each of the NC_FORMATX dispatchers to store in a way that is appropriate to the given dispatcher.

There is a complication with respect to the *nc_plugin_path_get* function. It is possible for users to bypass the netcdf API and modify the HDF5 plugin paths directly. This can result in an inconsistent plugin path between the value used by HDF5 and the global value used by netcdf-c. Since there is no obvious fix for this, we warn the user of this possibility and otherwise ignore it.

## Test Changes
* Two new tests
    a. unit_test/run_pluginpaths.sh -- was created to test this new capability.
    b. A new test utility has been added as *unit_test/run_dfaltpluginpath.sh* to test the default plugin path list.

## Documentation
* A new file -- docs/pluginpath.md -- provides documentation of the new API.

## Misc. Changes
1. Add some path manipulation utilities to netcf_aux.h
2. Fix the construction of netcdf_json.h as a BUILT_SOURCE
3. Fix some minor bugs in netcdf_json.h
4. Convert netcdf_json.h and netcdf_proplist.h to BUILT_SOURCE.
5. Add NETCDF_ENABLE_HDF5 as synonym for USE_HDF5
6. Fix some size_t <-> int conversion warnings.
7. Encountered and fixed the Windows \r\n problem in tst_pluginpaths.c.
8. Cleanup some minor CMakeLists.txt problems.

## Addendum: Proposed API

The API makes use of a counted vector of strings representing the sequence of directories in the path. The relevant type definition is as follows.
````
typedef struct NCPluginList {size_t ndirs; char** dirs;} NCPluginList;
````

The API proposed in this PR looks like this (from netcdf-c/include/netcdf_filter.h).

* ````int nc_plugin_path_ndirs(size_t* ndirsp);````
    Arguments: *ndirsp* -- store the number of directories in this memory.

    This function returns the number of directories in the sequence if internal directories of the internal plugin path list.

* ````int nc_plugin_path_get(NCPluginList* dirs);````
    Arguments:  *dirs* -- counted vector for storing the sequence of directies in the internal path list.

    This function returns the current sequence of directories from the internal plugin path list. Since this function does not modify the plugin path, it does not need to be locked; it is only when used to get the path to be modified that locking is required.  If the value of *dirs.dirs* is NULL (the normal case), then memory is allocated to hold the vector of directories. Otherwise, use the memory of *dirs.dirs* to hold the vector of directories.

* ````int nc_plugin_path_set(const NCPluginList* dirs);````
    Arguments: *dirs* -- counted vector for providing the new sequence of directories in the internal path list.

    This function empties the current internal path sequence and replaces it with the sequence of directories argument. Using an *ndirs* argument of 0 will clear the set of plugin paths.

RELEASE_NOTES.md

@@ -7,7 +7,7 @@ This file contains a high-level description of this package's evolution. Release
 
 ## 4.9.3 - TBD
 
-* Extend the netcdf API to support programmatic changes to the plugin search path. See [Github #3033](https://github.com/Unidata/netcdf-c/pull/3033) for more information.
+* Extend the netcdf API to support programmatic changes to the plugin search path. See [Github #3034](https://github.com/Unidata/netcdf-c/pull/3034) for more information.
 
 ## Known Issue