Improve FreeBSD support (#33162)

This PR contains a set of changes for improving FreeBSD support (#15309,
#29550) and is a kind of follow up to the PR #20480 which added an
initial support for FreeBSD.

A summary of changes is as follows:
- Add some more freebsd conditionals which seem missing in the previous
PR.
- Implement `anonymous_fd()` and `current_path()` functions for FreeBSD.
- Improve detection of FreeBSD in telemetry and GPU detection.
- Temporarily disable LiveKit/WebRTC support to make build succeed.
- Remove support for flatpak since it is Linux-only packaging format.

Adding `RUSTFLAGS="-C link-dead-code"` does not seem necessary anymore.
It builds fine without the flag.

Known issues:
- Integrated terminal is painfully laggy and virtually unusable in my
environment. This might be specific to my setup.
- I cannot input Japanese using IME. When I type characters, they appear
on the screen. But when I hit return key, they disappears. Seems the
same issue as #15409.

My environment is MATE desktop on X11 on FreeBSD 14.2 on Intel Core
i5-7260U integrated graphics.

P.S. For those who might be interested, a work-in-progress FreeBSD port
and binary packages are available at
https://github.com/tagattie/FreeBSD-Zed

Release Notes:

- N/A

---------

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/livekit_client/Cargo.toml

@@ -39,7 +39,7 @@ tokio-tungstenite.workspace = true
 util.workspace = true
 workspace-hack.workspace = true
 
-[target.'cfg(not(all(target_os = "windows", target_env = "gnu")))'.dependencies]
+[target.'cfg(not(any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")))'.dependencies]
 libwebrtc = { rev = "80bb8f4c9112789f7c24cc98d8423010977806a6", git = "https://github.com/zed-industries/livekit-rust-sdks" }
 livekit = { rev = "80bb8f4c9112789f7c24cc98d8423010977806a6", git = "https://github.com/zed-industries/livekit-rust-sdks", features = [
     "__rustls-tls"

crates/livekit_client/src/lib.rs

@@ -6,32 +6,32 @@ pub use remote_video_track_view::{RemoteVideoTrackView, RemoteVideoTrackViewEven
 #[cfg(not(any(
     test,
     feature = "test-support",
-    all(target_os = "windows", target_env = "gnu")
+    any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")
 )))]
 mod livekit_client;
 #[cfg(not(any(
     test,
     feature = "test-support",
-    all(target_os = "windows", target_env = "gnu")
+    any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")
 )))]
 pub use livekit_client::*;
 
 #[cfg(any(
     test,
     feature = "test-support",
-    all(target_os = "windows", target_env = "gnu")
+    any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")
 ))]
 mod mock_client;
 #[cfg(any(
     test,
     feature = "test-support",
-    all(target_os = "windows", target_env = "gnu")
+    any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")
 ))]
 pub mod test;
 #[cfg(any(
     test,
     feature = "test-support",
-    all(target_os = "windows", target_env = "gnu")
+    any(all(target_os = "windows", target_env = "gnu"), target_os = "freebsd")
 ))]
 pub use mock_client::*;