MFC r345350, r346441, r346765

r345350:
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
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19649

r346441:
Use symlinks for kernel modules rather than hardlinks

When aliasing a kernel module to a different name (ie if_igb for if_em),
it's better to use symlinks than hard links. kldxref will omit entries for
the links, ensuring that the loaded module has the correct name.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19979

r346765:
Don't symlink fusefs.ko to fuse.ko on PPC

Some PPC systems (PowerNV) use msdosfs for /boot, which can't handle either
symlinks or hardlinks. So on PPC, copy the module instead. This change fixes
installkernel on such systems after r345350.

Reported by: Brandon Bergren <git_bdragon.rtk0.net>
Reviewed by: jhibbits, rgrimes
MFC-With: 345350, 346441
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19993