lib/syslinux: Port to fd-relative
lib/syslinux: Port to fd-relative

Prep for `ostree_sysroot_new_at()`.  Also just generally nicer.

Closes: #1237
Approved by: jlebon

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