Enable automounting of exFAT media.

With fstyp(8) being updated to detect exfat in base r312003, it seems
like a good time to add support for auto-mounting SDXC cards -- which
use exfat by default.

The user will need to locally compile and install sysutils/fusefs-exfat
for this to succeed; logs a message to that effect when not installed.

PR: 218743
Submitted by: eborisch+FreeBSD@gmail.com
MFC after: 2 weeks