This change makes public the current kargs API in src/libostree/ostree-kernel-args.c
and adds documentations.
Upstreams the new kargs API from rpm-ostree/src/libpriv/rpmostree-kargs-process.c
Merges libostree_kernel_args_la_SOURCES to libostree_1_la_SOURCES in Makefile-libostree.am
Upstreams tests/check/test-kargs.c from rpm-ostree.
Closes: #1833Closes: #1869
Approved by: jlebon
Allen Bai