ostree admin instutil set-kargs: make more flexible

Add command line arguments:
 --import-proc-cmdline: import values from /proc/cmdline
 --merge: import current values
 --replace=ARG=VALUE: replace value
 --append=ARG=VALUE: append a new argument

Extra command line arguments are treated like --append=, which
gives backwards compatibility.

https://bugzilla.gnome.org/show_bug.cgi?id=731051

src/libostree/ostree-kernel-args.h

@@ -40,6 +40,10 @@ void _ostree_kernel_args_append (OstreeKernelArgs  *kargs,
 void _ostree_kernel_args_append_argv (OstreeKernelArgs  *kargs,
                                       char **argv);
 
+gboolean _ostree_kernel_args_append_proc_cmdline (OstreeKernelArgs *kargs,
+                                                  GCancellable     *cancellable,
+                                                  GError          **error);
+
 void _ostree_kernel_args_parse_append (OstreeKernelArgs *kargs,
                                        const char *options);