Provide proper ca_table for carrier unit# selectors

Provide proper ca_table for carrier unit# selectors

Makes ef_verify() check carrier UIDs are sane.  Partially protects
unit_cargo_init(), which oopses on bad carriers.

This has become possible only since commit 64a53c90 (v4.3.17) set
their values to -1 in newly created units.  Before, they were zero in
units that had never been used, and a proper ca_table would have made
ef_verify() fail when unit#0 didn't exist.

The only unit# selectors left without a proper ca_table are ship's
follow, lost's id and trade's unitid.  Document why.