filesystems/gvfs: Use fuser(1) in place of lsof(1) by default

• Import patch accepted upstream to allow using fuser(1) to check for busy mountpoints [1]
• Add option to allow choosing between fuser and lsof (fuser by default

This removes an hard dependency on lsof for this very commonly used
port.

[1] https://gitlab.gnome.org/GNOME/gvfs/-/merge_requests/321

Reviewed by: gnome (fluffy)
Approved by: gnome (fluffy)
Differential Revision: https://reviews.freebsd.org/D56224