configs/releng/syslinux/archiso_pxe.cfg: add "checksum" and "verify" boot options

An ISO's checksum and GPG validation primarily matters when PXE booting, so it would be appropriate to enable these options for releng. They should protect against booting a corrupt image.

archiso/initcpio/hooks/archiso

@@ -14,7 +14,7 @@ _mnt_dmsnapshot() {
 
     ro_dev="$(losetup --find --show --read-only -- "${img}")"
     echo "${ro_dev}" >> /run/archiso/used_block_devices
-    ro_dev_size="$(blockdev --getsz -- "${ro_dev}")"
+    ro_dev_size="$(blockdev --getsz "${ro_dev}")"
 
     if [ "${cow_persistent}" = "P" ]; then
         if [ -f "/run/archiso/cowspace/${cow_directory}/${img_name}.cow" ]; then