FreeBSD: deprecate TIOCMGDTRWAIT and TIOCMSDTRWAIT

These constants were removed on `main` in ec605ff ("libc: fix ctest
failure on freebsd15"). Mark them as deprecated with 0.2 rather than
removing directly.

Effectively a backport of #4685.

Author: tgross35

src/unix/bsd/freebsdlike/mod.rs

@@ -1238,7 +1238,15 @@ pub const TIOCGETD: c_ulong = 0x4004741a;
 pub const TIOCSETD: c_ulong = 0x8004741b;
 pub const TIOCGDRAINWAIT: c_ulong = 0x40047456;
 pub const TIOCSDRAINWAIT: c_ulong = 0x80047457;
+#[cfg_attr(
+    not(target_os = "dragonfly"),
+    deprecated = "unused since FreeBSD 8, removed in FreeBSD 15"
+)]
 pub const TIOCMGDTRWAIT: c_ulong = 0x4004745a;
+#[cfg_attr(
+    not(target_os = "dragonfly"),
+    deprecated = "unused since FreeBSD 8, removed in FreeBSD 15"
+)]
 pub const TIOCMSDTRWAIT: c_ulong = 0x8004745b;
 pub const TIOCDRAIN: c_ulong = 0x2000745e;
 pub const TIOCEXT: c_ulong = 0x80047460;