tool_main.c: post VMS patch cleanup - I

- remove header inclusion already done in curl_setup_once.h
vms_show: post VMS patch cleanup - II

- remove multiple declarations of vms_show and add comments
setup-vms.h: post VMS patch cleanup - III

- rename post-config-vms.h to setup-vms.h
- move its inclusion into proper location in curl_setup.h