Add support for libfuse3 (sysutils/fusefs-libs3).

USES=fuse:3 does add a dependency on version 3 of libfuse, which is not
compatible with version 2 (which remains default version for now). Both
versions can co-exist at build and run time.

Reviewed by: mat
Approved by: mat (portmgr)
Differential Revision: https://reviews.freebsd.org/D12694