curl_multibyte: fixup low-level calls, include in unity builds

[vszakats]

Also adjust () around low-level calls preventing macro overrides via
e.g. memdebug.h:

• add for malloc and free.
• drop for _open. (We do not override _open in curl.)

Tidy-up: also sync libcurlu custom macro order in cmake with autotools.

Follow-up to f42a279 #11928

[jay]

The comment at the head of this file should be updated to explain why all functions in the file must now call the original versions of memory functions. For example,.

/*
 * The functions in this file are curlx functions which are not tracked by the
 * curl memory tracker memdebug so they do not use the macro replacements
 * free, malloc, etc. Instead wrap the names in parentheses to call the
 * original versions: `ptr = (malloc)(123)`, `(free)(ptr)`, etc.
 */

[vszakats]

That comment could use an update indeed. I also learned about test 1132,
which hopefully may be unnecessary one day.

I guess the root reason we don't want to call the debug allocators from these
functions, is because these might be called by the debug allocators. The
memdebug.h requirement assumes a per-file compilation model, which is
no longer true with unity and bundle builds. I think the root-root cause is
redefining system symbols, and the () trick serves as a workaround (with
caveats).

Updated the comment!