Instead of splitting a string of kargs with a space as the
separator, parse the string and allow spaces that are protected
by quotes in the karg value.

The next_arg() function is based on the implementation in the
Linux kernel: https://github.com/torvalds/linux/blob/8a05452ca460b05c985eadc7b5a4f040f124463e/lib/cmdline.c#L204

Add the following config directories for setting default kernel
arguments:

/etc/ostree/kargs.d (host config)
/usr/lib/ostree-boot/kargs.d (base config)

These directories contain files whose contents consist of a karg
snippet, which is a collection kernel parameter keys and values.
Example of a snippet:

```
KEY=FOO KEYFLAG ANOTHERKEY=VALUE1 ANOTHERKEY=VALUE2
```

Snippets are read in alphanumeric order of the karg snippet filename
when generating the final kernel options from the host and base config
directories. Ordering may be specified by prefixing a number, e.g.
`/etc/ostree/kargs.d/4000_localhost`.

The bootconfig key `ostree-kargs-generated-from-config` indicates
whether the kargs were generated by ostree from the kargs.d directories
(`true`), or copied from the previous deployment (`false`). Editing the
kargs through the command line (e.g. `ostree admin deploy --karg=`) will
set the flag to `false`, and kargs will be copied from the previous
deployment for subsequent deployments.

Also add support for `ostree admin instutil set-kargs` so that installer
programs using this command (such as Anaconda) remain managed by the
kargs.d directories from the first deployment.

Closes: ostreedev#479