Allow configuring the user, group and mode of the rfb unix socket to
make guest console access management easier. By default, the socket
remains created with the current bhyve process user and their group
as owners and a default mode of 0600.
Sponsored by: Defenso
MFC after: 2 weeks
Signed-off-by: Quentin Thébault <quentin.thebault@defenso.fr>