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/client/src/telemetry.rs

@@ -83,10 +83,14 @@ pub fn os_name() -> String {
     {
         "macOS".to_string()
     }
-    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
+    #[cfg(target_os = "linux")]
     {
         format!("Linux {}", gpui::guess_compositor())
     }
+    #[cfg(target_os = "freebsd")]
+    {
+        format!("FreeBSD {}", gpui::guess_compositor())
+    }
 
     #[cfg(target_os = "windows")]
     {
@@ -120,8 +124,12 @@ pub fn os_version() -> String {
             file
         } else if let Ok(file) = std::fs::read_to_string(&Path::new("/usr/lib/os-release")) {
             file
+        } else if let Ok(file) = std::fs::read_to_string(&Path::new("/var/run/os-release")) {
+            file
         } else {
-            log::error!("Failed to load /etc/os-release, /usr/lib/os-release");
+            log::error!(
+                "Failed to load /etc/os-release, /usr/lib/os-release, or /var/run/os-release"
+            );
             "".to_string()
         };
         let mut name = "unknown";