feature request: support specifying Unix socket with systemd directive OpenFile=

Hello,
curl already has the command-line option

--unix-socket path

where the UNIX socket is specified as a path in the file system.
It would be nice if curl would also support using an inherited file
descriptor that originates from systemd.

The systemd directive OpenFile= was introduced in systemd 253
(released February 2023).

Man page:
https://www.freedesktop.org/software/systemd/man/systemd.service.html#OpenFile=

Rationale: Due to restrictive file permissions on a Unix socket, curl
might not be able to connect to the Unix socket of a web server. By
letting systemd connect to the Unix socket and then letting curl
inherit the file descriptor, curl would be able to retrieve web pages
from the Unix socket.

Demo:
I wrote a minimal demo where I modified docs/examples/externalsocket.c
to make use of a Unix socket specified with OpenFile=

https://github.com/eriksjolund/podman-OpenFile
https://github.com/eriksjolund/curl/blob/externalsocket_openfile/docs/examples/externalsocket.c

Regarding the user interface, a new command-line argument could be added

For example:
--unix-socket-fdname myfdname

A side note: OpenFile= could also be used for opening normal files.
In addition to the already existing command-line argument
--proxy-cacert
a new command-line argument
--proxy-cacert-fdname myfdname
could be added.

Kind regards,
Erik Sjölund
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