MFC r335479, r335509

MFC r335479: subr_hints: simplify a little bit

Some complexity exists in these bits that isn't needed. The sysctl handler,
upon change to '2', runs through the current set of hints and sets them in
the kenv.

However, this isn't at all necessary if we're pulling hints from the kenv,
static or dynamic, as the former will get added to the latter in
init_dynamic_kenv (see: kern_environment.c). We can reduce this
configuration to just adding static_hints to the kenv if we were previously
using them.

The changes in res_find are minimal and based on the observation that once
use_kenv gets set to '1' it will never be reset to '0', and it gets set to
'1' as soon as we hit fallback mode. Later work will refactor res_find a
little bit and eliminate this now-local, because it's become clear that
there's some funkiness revolving around use_kenv=1 and it being used to
imply that we're certainly looking at the dynamic_kenv.

MFC r335509: subr_hints: Fix acpi unit hinting (at the very least)

The refactoring in r335479 overlooked the fact that the dynamic kenv can
also be switched to if hintmode == 0. This is problematic because the
checkmethod bits are only ever ran once, but it worked previously because
the use_kenv was a global state and the first lookup would enable it if
occurring after the dynamic environment has been setup.

Extending our local definition of use_kenv to include all non-STATIC
hintmodes as long as the dynamic_kenv is setup fixes this. We still have
potential issues if the dynamic kenv comes up while we're doing an anchored
search through the environment, but this is not much of a concern right now
because:

1.) The dynamic environment comes up super early in boot, just after kmem

2.) This is going to get rewritten to provide a safer mechanism for the
anchored searches, ensuring that we continue using the same environment
chain (dynamic env or static fallback) for all anchored search invocations