Add an internal libiscsiutil library.

Move some of the code duplicated between ctld(8) and iscsid(8) into a
libiscsiutil library.

Sharing the low-level PDU code did require having a
'struct connection' base class with a method table to permit separate
initiator vs target behavior (e.g. in handling proxy PDUs).

Reviewed by: mav, emaste
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D33544

(cherry picked from commit 6378393308bc6bd81fb871dacf6b03cf1a390d8b)