gtk: revamp cgroup/transient scope handling

This changes the way Ghostty assigns itself and subprocesses to
cgroups and how resource controls are applied.

* Ghostty itself no longer modifies it's own cgroup or moves itself
to a transient scope. To modify the main Ghostty process' resource
controls ensure that you're launching Ghostty with a systemd unit and
use the standard systemd methods for overriding and applying changes
to systemd units.

* If configured (on by default), the process used to run your command
will be moved to a transient systemd scope after it is forked from
Ghostty but before the user's command is executed. Resource controls
will be applied to the transient scope at this time. Changes to
the `linux-cgroup*` configuration entries will not alter existing
commands. If changes are made to the `linux-cgroup*` configuration
entries commands will need to be relaunched. Resource limits can also
be modified after launch outside of Ghostty using systemd tooling. The
transient scope name can be shown by running `systemctl --user whoami`
in a shell running inside Ghostty.

Fixes #2084.
Related to #6669

src/Surface.zig

@@ -636,16 +636,8 @@ pub fn init(
             .working_directory = config.@"working-directory",
             .resources_dir = global_state.resources_dir.host(),
             .term = config.term,
-
-            // Get the cgroup if we're on linux and have the decl. I'd love
-            // to change this from a decl to a surface options struct because
-            // then we can do memory management better (don't need to retain
-            // the string around).
-            .linux_cgroup = if (comptime builtin.os.tag == .linux and
-                @hasDecl(apprt.runtime.Surface, "cgroup"))
-                rt_surface.cgroup()
-            else
-                Command.linux_cgroup_default,
+            .rt_pre_exec_info = .init(config),
+            .rt_post_fork_info = .init(config),
         });
         errdefer io_exec.deinit();