Fix regression on M110 command (kliment#1454)

 * Add test file for repeated prints
 * Add old behavior of M110 back and combine it with the actual behavior of Prusa firmware >3.12.x. Actual Prusa firmware >3.12.x needs "M110 N-1", Smoothieware firmware needs "N-1 M110" and old Marlin firmware 1.0.0 needs in addition line checksum. `self._send("M110 N-1", -1, True)` results in sending "N-1 M110 N-1*125" and does not harm Smoothieware and old Marlin firmware.

Fixes kliment#1421

printrun/printcore.py

@@ -416,7 +416,7 @@ def startprint(self, gcode, startindex = 0):
             return True
 
         self.clear = False
-        self._send("M110 N-1")
+        self._send("M110 N-1", -1, True)
 
         resuming = (startindex != 0)
         self.print_thread = threading.Thread(target = self._print,
@@ -675,7 +675,7 @@ def _sendnext(self):
             if not self.paused:
                 self.queueindex = 0
                 self.lineno = 0
-                self._send("M110 N-1")
+                self._send("M110 N-1", -1, True)
 
     def _send(self, command, lineno = 0, calcchecksum = False):
         # Only add checksums if over serial (tcp does the flow control itself)

testfiles/arc_test.gcode

@@ -0,0 +1,21 @@
+; Print this file for a quick test of support for arc movements with 
+; G2 and G3 and z helix movement up and down +- 10 mm, 6 times a full cycle
+; It takes only a half minute and prints above the print bed (z=20 mm)
+; w/o heating and extrusion
+
+G28 ; home
+G0 Z10.0000 S0 ; lift z
+G90 ; absolute mode
+G21	; set unit to mm
+
+G2 X15 Y20 I30 J40 Z20 F8000
+G3 X15 Y20 I30 J40 Z10 F8000
+G2 X15 Y20 I30 J40 Z20 F8000
+G3 X15 Y20 I30 J40 Z10 F8000
+G2 X15 Y20 I30 J40 Z20 F8000
+G3 X15 Y20 I30 J40 Z10 F8000
+
+G0 X5.0000
+G0 Y5.0000
+
+M84; motors off