Add the option "--disable-network-access" (automake)
or "-DENABLE_NETWORK_ACCESS=OFF" (cmake).
When disabled, this option transitively disables all
network access capabilities and testing.
If set, this option implies the following:
* --disable-dap
* --disable-byterange
* --disable-s3

This PR answers a request for a feature from Ed Hartnett.

## Misc. Other changes
* Take the opportunity to clean up some old, unused options;
e.g. --enable-multifilters.
* Fix bug in using S3 urls.

…s in nc-config, and I can see where people might want to use this define to write code that works with modern and older versions of netCDF.

…ood, did it refer to networked drives? Other things that are network adjacent?