sysroot: Also maintain canonical boot_fd

Just like we hold a fd for `/sysroot`, also do so for `/boot`
instead of opening and closing it in a few places.

This is a preparatory cleanup for further work.

src/libostree/ostree-sysroot-private.h

@@ -56,6 +56,7 @@ struct OstreeSysroot {
 
   GFile *path;
   int sysroot_fd;
+  int boot_fd;
   GLnxLockFile lock;
 
   OstreeSysrootLoadState loadstate;
@@ -160,6 +161,9 @@ char * _ostree_sysroot_get_runstate_path (OstreeDeployment *deployment, const ch
 
 char *_ostree_sysroot_join_lines (GPtrArray  *lines);
 
+gboolean
+_ostree_sysroot_ensure_boot_fd (OstreeSysroot *self, GError **error);
+
 gboolean _ostree_sysroot_query_bootloader (OstreeSysroot     *sysroot,
                                            OstreeBootloader **out_bootloader,
                                            GCancellable      *cancellable,