Address #8573, #8595

-Report SERIAL_XON_XOFF in Extended Capabilites M115 report for hosts
-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines
-Update error message conditions

More corrections to make that happen...

G26 uses logic that looks like:
```
ex = _GET_MESH_X(i + 1) - (SIZE_OF_INTERSECTION_CIRCLES - (SIZE_OF_CROSSHAIRS));
```
The Bi-Linear macros need parenthesis to work correctly.

[2.0.x] Fix missing return values in G33_auto_tune

[2.0] Skew Correction

[2.0.x] Fix some planner bugs

Fix M49 so that it does not hang the terminal console.

[2.0] led_control_menu

[2.0.x] Segmented mesh moves

Remove duplicates, with followup fixes

[2.0.x] Additional Travis BABYSTEPPING tests

[2.0.x] Fix WDTO during long delta moves

Fix #8577

Fan speed code fell into a curly brace

During long lasting moves of a delta machine neither the temperatures are maintained nor buttons checked nor ... .
The fix is to call 'manage_heater()' and 'idle()' every now and then in the loop where the move is segmented. 
The code is abot the same as in G2_G3 wher we had the same problem with too large circles lasting too long.
Fixing #7510