MFC r313959:

Add USB Mass Storage CTL frontend. This makes it possible
for USB OTG-capable hardware to implement device side of USB
Mass Storage, ie pretend it's a flash drive. It's configured
in the same way as other CTL frontends, using ctladm(8)
or ctld(8). Differently from usfs(4), all the configuration
can be done without rebuilding the kernel.

Testing and review is welcome. Right now I'm still moving,
and I don't have access to my test environment, so I'm somewhat
reluctant to making larger changes to this code; on the other
hand I don't want to let it sit on Phab until my testing setup
is back, because I want to get it into 11.1-RELEASE.

Relnotes: yes
Sponsored by: The FreeBSD Foundation