rsu(4) does not require legal.realtek.license_ack=1

The rsu firmware license check has been disabled since r292756. Changes
rsu(4) since the license ack is no longer required.

While here, add device rsufw hint to the kernel configuration lines and
add/update paths to the installed license file in both rsu(4) and rsufw(4).

Submitted by: Mateusz Piotrowski (0mp)
Reviewed by: bcr (manpages)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D14966