special-case getvfsbyname(3) for fusefs(5)

fusefs file systems may have a fsname subtype (set by mount_fusefs's "-o
subtype" option) that gets appended to the fsname as returned by statfs(2).
The subtype is set on a per-mount basis so it isn't part of the struct
vfsconf. Special-case getvfsbyname to match either the full "fusefs.foobar"
or short "fusefs" fsname.

This is a merge of r348007, r348054, and r350093 from projects/fuse2

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21043