tcp: add SO_LINGER socket option (Linux #13, mapped to #14)

SO_LINGER controls close() behavior via struct linger
  { int l_onoff; int l_linger; }
  - get returns {1, 0} (linger enabled, zero timeout)
  - set accepts any value, delegates close semantics to smoltcp

Uses constant 14 since the flat Redox namespace can't use
Linux level SOL_SOCKET=13 (collides with TCP_CONGESTION=13).

All 31 tests pass.

netstack/src/scheme/tcp.rs

@@ -17,6 +17,7 @@ const SO_SNDBUF: usize = 7;
 const SO_RCVBUF: usize = 8;
 const SO_KEEPALIVE: usize = 9;
 const SO_REUSEADDR: usize = 2;
+const SO_LINGER: usize = 14;
 const TCP_NODELAY: usize = 1;
 const TCP_MAXSEG: usize = 2;
 const TCP_KEEPIDLE: usize = 4;
@@ -538,6 +539,16 @@ impl<'a> SchemeSocket for TcpSocket<'a> {
                 buf[..len].copy_from_slice(&bytes[..len]);
                 Ok(len)
             }
+            SO_LINGER => {
+                // struct linger: l_onoff (4 bytes) + l_linger (4 bytes)
+                let vals = [1i32, 0i32]; // on, 0s linger
+                let bytes = unsafe {
+                    core::slice::from_raw_parts(vals.as_ptr() as *const u8, 8)
+                };
+                let len = buf.len().min(bytes.len());
+                buf[..len].copy_from_slice(&bytes[..len]);
+                Ok(len)
+            }
             // SO_REUSEADDR(2) collides with TCP_MAXSEG(2) — TCP_MAXSEG handles it
             _ => Err(SyscallError::new(syscall::ENOPROTOOPT)),
         }
@@ -591,6 +602,7 @@ impl<'a> SchemeSocket for TcpSocket<'a> {
                 Ok(1)
             }
             IP_MTU_DISCOVER => Ok(1),
+            SO_LINGER => Ok(1), // accepted, smoltcp handles close semantics
             // SO_REUSEADDR(2) collides with IP_TTL(2) — IP_TTL handles it
             _ => Err(SyscallError::new(syscall::ENOPROTOOPT)),
         }