kmod-setup: split warn flags

kmod-setup: split warn flags

Traditionally, we used to warn about ipv6 being a module or being
unavailable. This was changed in b4aa82f16 ("kmod-setup: don't warn
when ipv6 can't be loaded") in a way that neither of the two conditions
will cause a log message.

Now, while running a setup without any IPv6 is completely fine and
shouldn't cause any warning, we should still warn about ipv6 being a
module instead of built-in.

To achieve this, split the boolean warn flag into two: one for a
feature not being built-in but shipped as a module, and one to
print an error when a module is entirely unavailable.

We will, however, still warn if kmod returns anything else than
-ENOENT in the attempt of loading the module, and at the very least,
turn the message into a debug log.