Prevent Telnet peer disconnects from triggering the TCP watchdog

[figamore]

Problem

A stalled or dead Telnet peer could block TelnetClient::write() indefinitely,
starving the FreeRTOS TCP task watchdog and causing an unintended reboot.

Changes

• Replaced Telnet's blocking WiFiClient::write() with ::send(..., MSG_DONTWAIT) so writes never hold the CPU.
• Added a line-accumulation buffer so output is flushed one complete line at a time, keeping \r\n
  normalization correct across fragmented write() calls.
• When the client's TCP send buffer is full, the code waits up to TX_IDLE_WAIT_MS (50 ms) when the
  machine is idle, but drops immediately during motion to keep jogging/running unaffected.
• Clients that stall TX_STALL_LIMIT (10) consecutive lines are disconnected and reaped on the next
  poll(), freeing the slot for a healthy reconnect.