sysroot: Write symlinks before calling fsync(), then rename after

There might be a race here in that we create new symlink files *after*
calling `syncfs`, and they are not guaranteed to end up on disk.

Rework the code so that we create symlinks before, and then only
rename them after (and `fsync()` the directory for good measure).

Additional-fixes-by: Giuseppe Scrivano <gscrivan@redhat.com>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>

This still needs verification that we're fixing a real bug; but I'm
fairly confident this won't make the fsync situation worse.

https://bugzilla.gnome.org/show_bug.cgi?id=755595

src/libotutil/ot-gio-utils.h

@@ -85,11 +85,6 @@ gboolean ot_gfile_ensure_unlinked (GFile         *path,
                                    GCancellable  *cancellable,
                                    GError       **error);
 
-gboolean ot_gfile_atomic_symlink_swap (GFile          *path,
-                                       const char     *target,
-                                       GCancellable   *cancellable,
-                                       GError        **error);
-
 gboolean ot_util_ensure_directory_and_fsync (GFile         *dir,
                                              GCancellable  *cancellable,
                                              GError       **error);