GUACAMOLE-2307: Add opt-in raw text-output mode for terminal protocols (SSH/telnet/Kubernetes)

[ciroiriarte]

Implements GUACAMOLE-2307.

What

Adds an opt-in per-connection parameter, text-output, that tees the raw remote
PTY byte stream (including ANSI/CSI/OSC sequences) to an outbound Guacamole pipe
stream named STDOUT, for the SSH, telnet, and Kubernetes protocols. A
non-graphical / native CLI client can then present a terminal session as true
in-terminal text instead of decoding rasterized glyphs. Default off; existing
connections are unaffected.

Modes

• text-output=true — tee: raw bytes go to STDOUT and the graphical
  display is still rendered for browser users.
• text-output=raw — headless: graphical rendering is skipped entirely; bytes
  are delivered only via STDOUT.
• anything else, including false or omitted — disabled.

Design notes for review

• The tee is taken at the protocol source — each protocol's PTY read path,
  upstream of the terminal emulator — rather than via guac_terminal_echo(),
  which strips escape sequences.
• The pipe uses mimetype application/octet-stream (base64 blobs, binary-safe,
  charset-agnostic) and is allocated on the connection owner's user socket (an
  even, ack-routable index), not broadcast.
• Additive public API: guac_terminal_text_output_{open,write,flush,close}.
• Flow control: clients must ack every blob; the unacknowledged backlog is
  bounded by bytes (256 KB). On a stalled consumer, tee mode drops buffered
  output (its read loop is shared with the graphical display, so blocking would
  stall co-attached browser users), while raw mode throttles the read loop,
  propagating backpressure to the remote program through the PTY, and disconnects
  only after 15 s with no acks.
• Gated behind disable-copy: guacd refuses to open the pipe when copying from the
  terminal is disabled. Pipe contents are not logged or recorded by default.

Compatibility

Additive and opt-in, default off. No new protocol instruction (reuses
pipe/blob/end/ack), no schema change, no ABI break. An unmodified
guacamole-client already passes the parameter through.

Testing

• CUnit coverage for the text-output paths (disable-copy gating, ack routing,
  tee-drop, raw throttle/abort, the byte bound, disconnect). make check green on
  Debian 12 and openSUSE Leap 16.
• Direct-to-guacd manual e2e harness across SSH/telnet/Kubernetes
  (util/manual-tests/).
• Validated live through an unmodified guacamole-client gateway with a native CLI
  client in both modes, including a 20,000-line flood.

Coordinated PRs

• guacamole-client (UI: protocol parameter + translations): to follow
• guacamole-manual (documentation): to follow