Refactor command-line parsing

Refactor command-line parsing to better utilize GOptionContext.  This
eliminates most of the manual parsing and global options are now shown
in the help output.

Here's a sample:

    $ ostree admin --help
    Usage:
      ostree admin [OPTION...] --print-current-dir|COMMAND

    Builtin "admin" Commands:
      cleanup
      config-diff
      deploy
      init-fs
      instutil
      os-init
      status
      switch
      undeploy
      upgrade

    Help Options:
      -h, --help         Show help options

    Application Options:
      --sysroot=PATH     Create a new OSTree sysroot at PATH
      -v, --verbose      Print debug information during command processing
      --version          Print version information and exit

https://bugzilla.gnome.org/show_bug.cgi?id=740295

src/ostree/ot-builtin-checksum.c

@@ -22,6 +22,7 @@
 
 #include "config.h"
 
+#include "ot-main.h"
 #include "ot-builtins.h"
 #include "ostree.h"
 #include "libgsystem.h"
@@ -56,7 +57,7 @@ on_checksum_received (GObject    *obj,
 }
 
 gboolean
-ostree_builtin_checksum (int argc, char **argv, OstreeRepo *repo, GCancellable *cancellable, GError **error)
+ostree_builtin_checksum (int argc, char **argv, GCancellable *cancellable, GError **error)
 {
   GOptionContext *context;
   gboolean ret = FALSE;
@@ -64,9 +65,8 @@ ostree_builtin_checksum (int argc, char **argv, OstreeRepo *repo, GCancellable *
   AsyncChecksumData data = { 0, };
 
   context = g_option_context_new ("PATH - Checksum a file or directory");
-  g_option_context_add_main_entries (context, options, NULL);
 
-  if (!g_option_context_parse (context, &argc, &argv, error))
+  if (!ostree_option_context_parse (context, options, &argc, &argv, OSTREE_BUILTIN_FLAG_NO_REPO, NULL, cancellable, error))
     goto out;
 
   if (argc > 1)