MFC r273635, r273793, r274797: Add basic iSNS client to the iSCSI target.

This makes ctld(8) register its iSCSI targets and portals on configured
iSNS servers to allow initiators find them without active discovery.

Fetching of allowed initiators from iSNS is not implemented now, so target
ACLs still should be configured manually.

Relnotes:	Yes
Sponsored by:	iXsystems, Inc.
MFC r274407

  Fix incorrect reading of 32-bit modinfo by 64-bit loaders.

  The various structures in the mod_metadata set of a FreeBSD kernel and
  modules contain pointers. The FreeBSD loader correctly deals with a
  mismatch in loader and kernel pointer size (e.g. 32-bit i386/ppc
  loader, loading 64-bit amd64/ppc64 kernels), but wasn't dealing with
  the inverse case where a 64-bit loader was loading a 32-bit kernel.
MFC r273760:
Fix prototypes.