fs/msdosfs: add support for file namws with surrogate pairs

Long file names are using UTF-16 symbols to represent international or
special characters. The implementation in FreeBSD did not support the
"Supplementary Private Use Area-B" (PUA-B), which requires a surrogate
pair to be represented in UTF-16 (Unicode code points beyond U+FFFF).

The PUA-B is used to represent emoji characters, which are supported
in file names on other common operating systems. The motivation for
this change was that removable media written on another system were
only partially readable on FreeBSD, since they contained emojis in
file names.

A test script that verifies correct operations on files names with
emojis has been added to the tools/test/stress2/misc directory under
the name msdos24.sh.

Reported by: Fabian Keil <fk@fabiankeil.de>
Reviewed by: ib
Approved by: mkcusick
MFC after: 1 week
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D57313