Add support for the ps command on FreeBSD, NetBSD, and OpenBSD (#12892)

# Description

I feel like it's a little sad that BSDs get to enjoy almost everything
other than the `ps` command, and there are some tests that rely on this
command, so I figured it would be fun to patch that and make it work.

The different BSDs have diverged from each other somewhat, but generally
have a similar enough API for reading process information via
`sysctl()`, with some slightly different args.

This supports FreeBSD with the `freebsd` module, and NetBSD and OpenBSD
with the `netbsd` module. OpenBSD is a fork of NetBSD and the interface
has some minor differences but many things are the same.

I had wanted to try to support DragonFlyBSD too, but their Rust version
in the latest release is only 1.72.0, which is too old for me to want to
try to compile rustc up to 1.77.2... but I will revisit this whenever
they do update it. Dragonfly is a fork of FreeBSD, so it's likely to be
more or less the same - I just don't want to enable it without testing
it.

Fixes #6862 (partially, we probably won't be adding `zfs list`)

# User-Facing Changes
`ps` added for FreeBSD, NetBSD, and OpenBSD.

# Tests + Formatting
The CI doesn't run tests for BSDs, so I'm not entirely sure if
everything was already passing before. (Frankly, it's unlikely.) But
nothing appears to be broken.

# After Submitting
- [ ] release notes?
- [ ] DragonflyBSD, whenever they do update Rust to something close
enough for me to try it

crates/nu-command/src/default_context.rs

@@ -164,6 +164,9 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
         #[cfg(any(
             target_os = "android",
             target_os = "linux",
+            target_os = "freebsd",
+            target_os = "netbsd",
+            target_os = "openbsd",
             target_os = "macos",
             target_os = "windows"
         ))]

crates/nu-command/src/system/mod.rs

@@ -5,6 +5,8 @@ mod nu_check;
     target_os = "android",
     target_os = "linux",
     target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd",
     target_os = "macos",
     target_os = "windows"
 ))]
@@ -23,6 +25,8 @@ pub use nu_check::NuCheck;
     target_os = "android",
     target_os = "linux",
     target_os = "freebsd",
+    target_os = "netbsd",
+    target_os = "openbsd",
     target_os = "macos",
     target_os = "windows"
 ))]

crates/nu-command/src/system/ps.rs

@@ -2,20 +2,13 @@
 use itertools::Itertools;
 use nu_engine::command_prelude::*;
 
-#[cfg(all(
-    unix,
-    not(target_os = "freebsd"),
-    not(target_os = "macos"),
-    not(target_os = "windows"),
-    not(target_os = "android"),
-))]
+#[cfg(target_os = "linux")]
 use procfs::WithCurrentSystemInfo;
 use std::time::Duration;
 
 #[derive(Clone)]
 pub struct Ps;
 
-#[cfg(not(target_os = "freebsd"))]
 impl Command for Ps {
     fn name(&self) -> &str {
         "ps"
@@ -82,7 +75,6 @@ impl Command for Ps {
     }
 }
 
-#[cfg(not(target_os = "freebsd"))]
 fn run_ps(
     engine_state: &EngineState,
     stack: &mut Stack,
@@ -111,12 +103,7 @@ fn run_ps(
 
         if long {
             record.push("command", Value::string(proc.command(), span));
-            #[cfg(all(
-                unix,
-                not(target_os = "macos"),
-                not(target_os = "windows"),
-                not(target_os = "android"),
-            ))]
+            #[cfg(target_os = "linux")]
             {
                 let proc_stat = proc
                     .curr_proc