config(8): De-dupe hint/env vars within a single file

r335653 flipped the order in which hints/env files are concatenated to match
the order in which vars are processed by the kernel. This is the other
hammer to drop.

Use nv(9) to de-dupe entries within a single hint or env file, using the
latest value specified for a key. This leaves some duplicates if a variable
is specified in multiple hint/env files or via envvar in a kernel config,
but the reversed order of concatenation (from r335653) makes this a
non-issue as the latest-specified version will be seen first.

This change also silently rewrote hint bits to use the same sanitization
process that ian@ wrote for r335642. To the kernel, hints and env vars are
basically the same thing through early boot, then get merged into the
dynamic environment once kmem becomes available and the dynamic environment
is created. They should be subjected to the same restrictions.

libnv has been added to -legacy for the time being to support the build of
config(8) with the new cnvlist API.

Tested with: universe (11 host & 12 host)
MFC after: 1 month