sbin/newfs_msdos: add support for exFAT file systems

Add support for the creation of exFAT file systems, if invoked as
newfs_exfat.

Reviewed by: olce
Approved by: olce, mckusick
MFC after: 4 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D48727