systemd: Use binary channel for Terminal

Terminals are not defined/required to be UTF-8 only. Sometimes people
cat binary data to the terminal, or possibly files in other encodings.
These cannot be sent through `TEXT` websocket channels, and will also
trigger the additional checks from commit 1b15dcb.

Use a binary channel to avoid all that. XTerm.js gets along with this
just fine, and it ignores invalid data.

With this you can now even `cat /bin/true` and get something moderately
sensible, instead of no output at all.

Fixes #20791

pkg/systemd/terminal.jsx

@@ -34,7 +34,8 @@ const _ = cockpit.gettext;
                     "TERM=xterm-256color",
                 ],
                 directory: user.home || "/",
-                pty: true
+                pty: true,
+                binary: true,
             });
         }
 

test/verify/check-system-terminal

@@ -91,6 +91,13 @@ PROMPT_COMMAND='printf "\033]0;%s@%s:%s\007" "${USER}" "${HOSTNAME%%.*}" "${PWD/
 
         b.input_text('echo -e "1\\u0041"\n')
         wait_line(n + 3, '1A')
+        wait_line(n + 4, prompt)
+
+        # non-UTF8 data
+        m.execute(r"echo -e 'hello\xFF\x01\xFF\x02world' > " + self.vm_tmpdir + "/garbage.txt")
+        b.input_text(f'cat {self.vm_tmpdir}/garbage.txt\n')
+        wait_line(n + 5, 'helloworld')
+        wait_line(n + 6, prompt)
 
         # The '@' sign is in the default prompt
         b.wait_in_text(".terminal-title", '@')