announce server hostname in MANIFEST, display in client

Server reads its hostname via gethostname(2) and includes it as a
length-prefixed UTF-8 string at the start of the MANIFEST payload.

Client displays 'Server: <hostname> — <MAC>' in a label above the
upstream list. Both sides updated for the new MANIFEST format:
  [hostname_len:1][hostname:N][entries...]

Protocol version unchanged (still 1); MANIFEST payload layout change
is backward-incompatible but both sides ship together.

gatunad/src/upstream.rs

@@ -1,6 +1,20 @@
 //! Upstream table: cmdline parsing and MANIFEST entry construction.
 
-use crate::frame::{UpstreamEntry, PROTO_TCP, PROTO_UDP};
+use crate::frame::{Frame, UpstreamEntry, PROTO_TCP, PROTO_UDP};
+
+/// Read the system hostname via `gethostname(2)`.
+pub fn get_hostname() -> String {
+    let mut buf = [0u8; 256];
+    let ret = unsafe {
+        libc::gethostname(buf.as_mut_ptr() as *mut libc::c_char, buf.len())
+    };
+    if ret == 0 {
+        let len = buf.iter().position(|&b| b == 0).unwrap_or(buf.len());
+        String::from_utf8_lossy(&buf[..len]).into_owned()
+    } else {
+        String::new()
+    }
+}
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[allow(dead_code)]