This is essentially a copy of
ostreedev/ostree#1785 to our private copy of the
kargs API. Should really dedupe those...

The really confusing part though was that that patch was intended to fix
the `rpm-ostree kargs --append EMPTYKEY=` case (coreos#1706), yet the
`rpmostree kargs --append KEYWORD` case (coreos#1779) wasn't also fixed, even
though that same ostree patch clearly fixes and tests for that too.

To make a long story short, we were passing buggy kargs to ostree, which
before that patch, had itself buggy kargs parsing which conveniently
fixed back the kargs we passed for the `KEYWORD` case.

Closes: coreos#1779

That's the latest available in (f28, el8). Prep for using some new APIs
without tripping compiler warnings.

We don't need to modify the key we're passed, so just use `const`.

When we expect a test to not fail, let's `g_assert_no_error` first so
that if it fails, we get an exact printout of what error we encountered
when we expected none.

Note this patch only touches the *new* APIs that aren't part of
libostree.

Now that we can use `g_ptr_array_find_with_equal_func`, we can drop our
custom `_ostree_ptr_array_find`.

Also strengthen our handling of values everywhere to handle the `NULL`
case and properly support `KEYWORD` args. I ended up getting rid of
`_ostree_kernel_arg_query_status` in the process since it made that
assumption a lot and overall added more complexity than necessary.