VFAT-safe file and directory names cannot end with dot

This adds special handling of trailing dots to vfatPath(),
that are replaced with '_'. This doesn't affect '.' and '..'.

../Music/R.E.M./Monster -> ../Music/R.E.M_/Monster

Unit tests updated, minor bugs when vfatPath() is invoked
with a PathSeparatorBehaviour not matching the system
detected by improving unit tests and fixed.

BUG: 388465