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/fs/src/fs.rs

@@ -272,7 +272,7 @@ impl FileHandle for std::fs::File {
         Ok(path)
     }
 
-    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
+    #[cfg(target_os = "linux")]
     fn current_path(&self, _: &Arc<dyn Fs>) -> Result<PathBuf> {
         let fd = self.as_fd();
         let fd_path = format!("/proc/self/fd/{}", fd.as_raw_fd());
@@ -287,6 +287,27 @@ impl FileHandle for std::fs::File {
         Ok(new_path)
     }
 
+    #[cfg(target_os = "freebsd")]
+    fn current_path(&self, _: &Arc<dyn Fs>) -> Result<PathBuf> {
+        use std::{
+            ffi::{CStr, OsStr},
+            os::unix::ffi::OsStrExt,
+        };
+
+        let fd = self.as_fd();
+        let mut kif: libc::kinfo_file = unsafe { std::mem::zeroed() };
+        kif.kf_structsize = libc::KINFO_FILE_SIZE;
+
+        let result = unsafe { libc::fcntl(fd.as_raw_fd(), libc::F_KINFO, &mut kif) };
+        if result == -1 {
+            anyhow::bail!("fcntl returned -1".to_string());
+        }
+
+        let c_str = unsafe { CStr::from_ptr(kif.kf_path.as_ptr()) };
+        let path = PathBuf::from(OsStr::from_bytes(c_str.to_bytes()));
+        Ok(path)
+    }
+
     #[cfg(target_os = "windows")]
     fn current_path(&self, _: &Arc<dyn Fs>) -> Result<PathBuf> {
         anyhow::bail!("unimplemented")