config(8): Add compatibility shims for r335998

Plumb the %VERSREQ from Makefile.<arch> through to the rest of config(8).
We've recorded the config(8) version that we're calling "the end of
envmode and hintmode," and we'll write them out for earlier versions. Later
kernel version bumps will remove envmode/hintmode from the kernel as needed,
which is OK since the current kernel does not use them at all.

These compatibility shims really need to go away when the major version
rolls over...

Discussed with: imp