deploy: Also honor multiply-specified kernel args with existing config

If we deployed a new tree inside an existing OS, inheriting kernel
args, we need to use append() instead of replace() to avoid collapsing
multiply specified kernel arguments like console=/dev/foo
console=/dev/bar.

Reported-by: Dusty Mabe <dusty@dustymabe.com>
sysroot: Add precondition that load() has been called for some API calls

We require the caller to invoke load() at the moment to do most
things.
repo-pull: Allow pulling only one directory

Changes the pull API to allow pulling only a single directory instead
of the whole deployment.  This option is utilized by the check-diff
option in rpm-ostree.

Add a new state directory to hold <checksum>.commitpartial files, so
we know that we've only downloaded partial state.
fix pointer bug