deploy: Rework kernel arguments, add --karg-append to "admin deploy"

The "ordered hash" code was really just for kernel arguments.  And it
turns out it needs to be a multihash (for e.g. multiple console=
arguments).

So turn the OstreeOrderedHash into OstreeKernelArgs, and move the bits
to split key=value and such into there.

Now we're not making this public API yet - the public OstreeSysroot
just takes char **kargs.  To facilitate code reuse between ostree/ and
libostree/, make it a noinst libtool library.  It'll be duplicated in
the binary and library, but that's OK for now.  We can investigate
making OstreeKernelArgs public later.

https://bugzilla.gnome.org/show_bug.cgi?id=721136