[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This reverts commit 4475b8ca.  Further
testing revealed that the threaded resolver still uses a timeout of
0ms.  This revert however lowers the bound to a minimum of 1ms instead
of 10ms.

curl_multi_remove_handle() calls our socket function, and there's no
need to call curl_multi_socket_action().

This was a problem 9 years ago, and apparently, it has been fixed long
ago.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Fixes a crash that can occur due to recursion from InvalidateSockets()
to ReadInfo() to CurlRequest callbacks.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.