Use file-scoped lexicals for shared statement handles

Instead of passing around references, put re-usable SQL statement object
refs into the common hash %g.
Improve fork handling; support 0 exit-code for fork()-less OSes

Forking now closes standard IO handles, and is supported for status file
processing.

A new --zero flag is added to allow 0 exit-code in the face of failure.
This makes it possible to not deny client connections where fork() won't
work.
Usage output improvements