Rename fuse(4) to fusefs(4)
This makes it more consistent with other filesystems, which all end in "fs",
and more consistent with its mount helper, which is already named
"mount_fusefs".
Reviewed by: cem, rgrimes
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19649