Add M113 to get/set Host Keepalive

Marlin/Marlin_main.cpp

@@ -151,6 +151,7 @@
  * M110 - Set the current line number
  * M111 - Set debug flags with S<mask>. See flag bits defined in Marlin.h.
  * M112 - Emergency stop
+ * M113 - Get or set the timeout interval for Host Keepalive "busy" messages
  * M114 - Output current position to serial port
  * M115 - Capabilities string
  * M117 - Display a message on the controller screen
@@ -4385,6 +4386,27 @@ inline void gcode_M111() {
  */
 inline void gcode_M112() { kill(PSTR(MSG_KILLED)); }
 
+#if ENABLED(HOST_KEEPALIVE_FEATURE)
+
+  /**
+   * M113: Get or set Host Keepalive interval (0 to disable)
+   *
+   *   S<seconds> Optional. Set the keepalive interval.
+   */
+  inline void gcode_M113() {
+    if (code_seen('S')) {
+      host_keepalive_interval = (uint8_t)code_value_short();
+      NOMORE(host_keepalive_interval, 60);
+    }
+    else {
+      SERIAL_ECHO_START;
+      SERIAL_ECHOPAIR("M113 S", (unsigned long)host_keepalive_interval);
+      SERIAL_EOL;
+    }
+  }
+
+#endif
+
 #if ENABLED(BARICUDA)
 
   #if HAS_HEATER_1