lib/kargs: Make API public and upstream new rpm-ostree APIs

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: #1833

Closes: #1869
Approved by: jlebon

src/libostree/ostree-sysroot-private.h

@@ -23,7 +23,6 @@
 
 #include "libglnx.h"
 #include "ostree.h"
-#include "ostree-kernel-args.h"
 #include "ostree-bootloader.h"
 
 G_BEGIN_DECLS