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/feedback/src/system_specs.rs

@@ -133,7 +133,7 @@ impl Display for SystemSpecs {
 }
 
 fn try_determine_available_gpus() -> Option<String> {
-    #[cfg(target_os = "linux")]
+    #[cfg(any(target_os = "linux", target_os = "freebsd"))]
     {
         return std::process::Command::new("vulkaninfo")
             .args(&["--summary"])
@@ -152,7 +152,7 @@ fn try_determine_available_gpus() -> Option<String> {
             })
             .or(Some("Failed to run `vulkaninfo --summary`".to_string()));
     }
-    #[cfg(not(target_os = "linux"))]
+    #[cfg(not(any(target_os = "linux", target_os = "freebsd")))]
     {
         return None;
     }