FreeBSD compatibility patches (#11869)

# Description

nushell is verified to work on FreeBSD 14 with these patches.

What isn't supported on FreeBSD:
* the crate 'procfs' doesn't support FreeBSD yet, all functionality
depending on procfs is disabled
* several RLIMIT_* values aren't supported on FreeBSD - functions
related to these are disabled




# User-Facing Changes
n/a

# Tests + Formatting
n/a

# After Submitting
n/a

---------

Co-authored-by: sholderbach <sholderbach@users.noreply.github.com>

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

@@ -4,6 +4,7 @@ mod nu_check;
 #[cfg(any(
     target_os = "android",
     target_os = "linux",
+    target_os = "freebsd",
     target_os = "macos",
     target_os = "windows"
 ))]
@@ -20,6 +21,7 @@ pub use nu_check::NuCheck;
 #[cfg(any(
     target_os = "android",
     target_os = "linux",
+    target_os = "freebsd",
     target_os = "macos",
     target_os = "windows"
 ))]