MFH (r272173): offer to enable local_unbound after installation

Approved by:	re (gjb)
MFS r273191: MFC r273060:

Use write_psize instead of write_asize when doing vdev_space_update.
Without this change the accounting of L2ARC usage would be wrong and
give 16EB free space because the number became negative and overflows.

Approved by:	re (gjb)