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/gpui/src/platform/blade/blade_renderer.rs

@@ -421,7 +421,10 @@ impl BladeRenderer {
     /// Like `update_drawable_size` but skips the check that the size has changed. This is useful in
     /// cases like restoring a window from minimization where the size is the same but the
     /// renderer's swap chain needs to be recreated.
-    #[cfg_attr(any(target_os = "macos", target_os = "linux"), allow(dead_code))]
+    #[cfg_attr(
+        any(target_os = "macos", target_os = "linux", target_os = "freebsd"),
+        allow(dead_code)
+    )]
     pub fn update_drawable_size_even_if_unchanged(&mut self, size: Size<DevicePixels>) {
         self.update_drawable_size_impl(size, true);
     }