User could enable SERIAL_XON_XOFF on USB-native devices and it would not be enabled without warning, but M115 would report the capability as available.

1) add explanations as to why software SPIs are selected

2) remove unused flag/option

still trying to fix file name

This reverts commit 824980e70e, reversing
changes made to aa7efb96bf.

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-Remove outdated 256 size limitation from comments on RX_BUFFER_SIZE defines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.

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

Remove duplicates, with followup fixes

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