lib/kernel-args: Store kernel args as key/value entries

lib/kernel-args: Store kernel args as key/value entries

Define an `OstreeKernelArgsEntry` structure, which holds
both the key and the value. The kargs order array stores
entries for each key/value pair, instead of just the keys.
The hash table is used to locate entries, by storing
entries in a pointer array for each key. The same public
interface is preserved, while maintaining ordering
information of each key/value pair when
appending/replacing/deleting kargs.

Fixes: #1859