repo: Port sign_data() to libglnx tmpfile APIs
repo: Port sign_data() to libglnx tmpfile APIs

Part of dropping libgsystem.

Closes: #423
Approved by: giuseppe

gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree