Cleanup LUN addition/removal.

• Make ctl_add_lun() synchronous. Asynchronous addition was used by

Copan's proprietary code long ago and never for upstream FreeBSD.

• Move LUN enable/disable calls from backends to CTL core.
• Serialize LUN modification and partially removal to avoid double frees.
• Slightly unify backends code.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.