NOTE: it is important that this fix gets into 4.9.3
re: Issue https://github.com/Unidata/netcdf-c/issues/2798
## Modifications
* This PR includes PR https://github.com/Unidata/netcdf-c/pull/2813
* Support the following AWS environment variables in the internal S3 library
(they are already supported by aws-sdk-cpp).
- AWS_REGION
- AWS_DEFAULT_REGION
- AWS_ACCESS_KEY_ID
- AWS_CONFIG_FILE
- AWS_PROFILE
- AWS_SECRET_ACCESS_KEY
- (source https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html).
* Support an empty region when specifying s3.amazonaws.com as the host.
* Move some S3/AWS related functions to ds3util.c
* Add a test case to test empty region and AWS_[DEFAULT]_REGION.
`options->file` is allocated on the stack and so will never be `NULL`,
making this conditional always true. Instead, we want to check the
value of `file`. As `options` is allocated with `calloc`, we know that
`file` will be zero-initialised, and it's safe to just check the first
element
`chunk_size` is only used if `PRINT_CHUNK_WASTE_REPORT` is
defined. Also move the declaration of `chunk_size` inside the `#ifdef`
to silence `set-but-unused` warning
The most critical bug is in nch5s3comms.c.
I for some reason assumed that signing keys
did not contain any zero bytes. But obviously
it can, so a test was removed.
Other fixes:
1. Guarantee allocated memory is initialized to all zeros.
2. Cleanup errmsg handling in libncpoco.
3. Fix processing of aws list-objects-v2 because I misread the syntax.
