Switch to using a systemd generator for /var

If one wants to set up a mount for `/var` in `/etc/fstab`, it
won't be mounted since `ostree-prepare-root` set up a bind mount for
`/var` to `/sysroot/ostree/$stateroot/var`, and systemd will take
the already extant mount over what's in `/etc/fstab`.

There are a few options to fix this, but what I settled on is parsing
`/etc/fstab` in a generator (exactly like `systemd-fstab-generator` does),
except here we look for an explicit mount for `/var`, and if one *isn't* found,
synthesize the default ostree mount to the stateroot. Another nice property is
that if an admin creates a `var.mount` unit in `/etc` for example, that will
also override our mount.

Note that today ostree doesn't hard depend on systemd, so this behavior only
kicks in if we're built with systemd *and* libmount support (for parsing
`/etc/fstab`).  I didn't really test that case though.

Initially I started writing this as a "pure libc" program, but at one point
decided to use `libostree.so` to find the booted deployment. That didn't work
out because `/boot` wasn't necessarily mounted and hence we couldn't find the
bootloader config. A leftover artifact from this is that the generator code
calls into libostree via the "cmd private" infrastructure. But it's an easy way
to share code, and doesn't hurt.

Closes: #859
Approved by: jlebon

src/switchroot/ostree-system-generator.c

@@ -0,0 +1,67 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2017 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include "config.h"
+
+#include <err.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+
+#include "ostree-cmdprivate.h"
+#include "ostree-mount-util.h"
+
+static const char *arg_dest = "/tmp";
+static const char *arg_dest_late = "/tmp";
+
+/* This program is a simple stub that calls the implementation that
+ * lives inside libostree.
+ */
+int
+main(int argc, char *argv[])
+{
+  /* Important: if this isn't an ostree-booted system, do nothing; people could
+   * have the package installed as a dependency for flatpak or whatever.
+   */
+  { struct stat stbuf;
+    if (fstatat (AT_FDCWD, "/run/ostree-booted", &stbuf, 0) < 0)
+      exit (EXIT_SUCCESS);
+  }
+
+  if (argc > 1 && argc != 4)
+    errx (EXIT_FAILURE, "This program takes three or no arguments");
+
+  if (argc > 1)
+    arg_dest = argv[1];
+  if (argc > 3)
+    arg_dest_late = argv[3];
+
+  char *ostree_cmdline = read_proc_cmdline_ostree ();
+  if (!ostree_cmdline)
+    errx (EXIT_FAILURE, "Failed to find ostree= kernel argument");
+
+  { g_autoptr(GError) local_error = NULL;
+    if (!ostree_cmd__private__()->ostree_system_generator (ostree_cmdline, arg_dest, NULL, arg_dest_late, &local_error))
+      errx (EXIT_FAILURE, "%s", local_error->message);
+  }
+
+  exit (EXIT_SUCCESS);
+}