runtime: ensure management interface buffer can hold log buffer.

Otherwise we get weird edge cases where the network stack could try
to append to log buffer while management interface is holding
the log buffer and trying to push it out, and it's just no good.

The serialized log buffer at its maximum length is slightly longer
than 32 KiB, so we just allocate the largest possible TCP buffer
to the management interface to keep it simple.

artiq/firmware/runtime/lib.rs

@@ -176,7 +176,7 @@ pub extern fn main() -> i32 {
             panic!("out of memory");
         });
 
-        static mut LOG_BUFFER: [u8; 65536] = [0; 65536];
+        static mut LOG_BUFFER: [u8; 32768] = [0; 32768];
         logger_artiq::BufferLogger::new(&mut LOG_BUFFER[..]).register(startup);
         0
     }

artiq/firmware/runtime/mgmt.rs

@@ -69,7 +69,7 @@ fn worker(mut stream: &mut TcpStream) -> io::Result<()> {
 }
 
 pub fn thread(io: Io) {
-    let listener = TcpListener::new(&io, 4096);
+    let listener = TcpListener::new(&io, 65535);
     listener.listen(1380).expect("mgmt: cannot listen");
     info!("management interface active");