tarfs_mount allow control of vfs_mountedfrom

We default to passing the path of the tar file to vfs_mountedfrom
so we can tell where a filesystem was mounted from.
However this can make the output of mount(8) hard to read.

Allow things like:

mount -t tarfs -o as=basename $tar $tar /mnt

so "as" is recorded instead of $tar

Reviewed by: des
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D39273