curl: when allocating variables, add the name into the struct

This saves the name from being an extra separate allocation.

Closes #12891

tidy-up: mostly whitespace nits

- delete completed TODO from `./CMakeLists.txt`.
- convert a C++ comment to C89 in `./CMake/CurlTests.c`.
- delete duplicate EOLs from EOF.
- add

tidy-up: mostly whitespace nits

- delete completed TODO from `./CMakeLists.txt`.
- convert a C++ comment to C89 in `./CMake/CurlTests.c`.
- delete duplicate EOLs from EOF.
- add missing EOL at EOF.
- delete whitespace at EOL (except from expected test results).
- convert tabs to spaces.
- convert CRLF EOLs to LF in GHA yaml.
- text casing fixes in `./CMakeLists.txt`.
- fix a codespell typo in `packages/OS400/initscript.sh`.

Closes #11772

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tool: add "variable" support

Add support for command line variables. Set variables with --variable
name=content or --variable name@file (where "file" can be stdin if set
to a single

tool: add "variable" support

Add support for command line variables. Set variables with --variable
name=content or --variable name@file (where "file" can be stdin if set
to a single dash (-)).

Variable content is expanded in option parameters using "{{name}}"
(without the quotes) if the option name is prefixed with
"--expand-". This gets the contents of the variable "name" inserted, or
a blank if the name does not exist as a variable. Insert "{{" verbatim
in the string by prefixing it with a backslash, like "\\{{".

Import an environment variable with --variable %name. It makes curl exit
with an error if the environment variable is not set. It can also rather
get a default value if the variable does not exist, using =content or
@file like shown above.

Example: get the USER environment variable into the URL:

--variable %USER
--expand-url = "https://example.com/api/{{USER}}/method"

When expanding variables, curl supports a set of functions that can make
the variable contents more convenient to use. It can trim leading and
trailing white space with "trim", output the contents as a JSON quoted
string with "json", URL encode it with "url" and base 64 encode it with
"b64". To apply functions to a variable expansion, add them colon
separated to the right side of the variable. They are then performed in
a left to right order.

Example: get the contents of a file called $HOME/.secret into a variable
called "fix". Make sure that the content is trimmed and percent-encoded
sent as POST data:

--variable %HOME=/home/default
--expand-variable fix@{{HOME}}/.secret
--expand-data "{{fix:trim:url}}"
https://example.com/

Documented. Many new test cases.

Co-brainstormed-by: Emanuele Torre
Assisted-by: Jat Satiro
Closes #11346

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