If last file in directory has short filename and is preceded by a deleted long file name. SD menu will wrongly report long deleted file name as last entry

Fix for #6 3593, #3648
this time excludung 'DEL' instead of accidently '~'.

The character 0x7E is common in 8.3 filenames. Maybe the last condition
should be removed completely instead.

Don't mangel 8.3-filenames with chars > 0x7f

Windows produces 8.3filenames wit chars > 0x7f. Those have been rejected
by Marlin until now.

With these 'malformed' filenames can now be worked with:
In the LCD menue
With RepetierHost (V1.6.1 tested) - full support. Characters are
displayed as '?'
With Octoprint (1.2.10 tested) the files do not appear in the files
area. At the console, listed with M20 they appear with a '�'.
With Pronterface the files appear in the sd-window but you can't start
them. They are mangled by pronterface. The names are altered and than
recected by Marlin. In the console they apper with differen but not the
correct characters.

All in all a little step forward.

Fix for #3593

Following up on #3231

Apply `constrain`, `NOMORE`, `NOLESS` and `CRITICAL_SECTION` macros
wherever possible.

Keep "astyled" reformatting

Apply `ENABLED` / `DISABLED` macros to files needing only a small
number of changes.

Replace instances of the number 13 with FILENAME_LENGTH where
appropriate.

Replace instances of the number 13 with FILENAME_LENGTH where
appropriate.

Add serial checkRx in stepper ISR.
Copied HardwareSerial to MarlinSerial (Needed for checkRx).