mapchars: map invalid chars to remap range (win32)

[ThomasWaldmann]

This might be useful if we encounter strange paths within borg archives using borg on win32:

https://www.kernel.org/doc/html/latest/admin-guide/cifs/usage.html

mapchars

Translate six of the seven reserved characters (not backslash): *?<>|: to the remap range (above 0xF000), which also allows the CIFS client to recognize files created with such characters by Windows’s POSIX emulation. This can also be useful when mounting to most versions of Samba (which also forbids creating and opening files whose names contain any of these seven characters). This has no effect if the server does not support Unicode on the wire.

[ThomasWaldmann]

Fixed in master by #9264.