net/ucx: Improve FreeBSD ucx_vfs support
- replace Linux-specific fusermount3 with mount_fusefs/umount
- use FreeBSD-compatible FUSE capability detection
- fix UNIX socket ancillary data handling for SCM_RIGHTS and peer credentials on FreeBSD
This allows ucx_vfs to start and communicate correctly on FreeBSD.
PR: 297093
Reported-by: Laurent Chardon <laurent@FreeBSD.org>
Reviewed by: thierry (mentor)
Approved by: thierry (mentor)