Add getty/gettytab capability to generate an initial message dynamically.

[dab]

Summary

This modification adds a new gettytab(5) option (iM) to specify a program to run in order to generate the initial (banner) message.

Test Plan

Install modified getty on system. Modify /etc/gettytab to specify the iM option. Logout. Get new login prompt. Verify that the login prompt is preceded by the string produced by the program specified for the iM option.

[dab]

Example run:

Wed Aug 23 10:02:42 CDT 2017: iM file

FreeBSD/amd64 (localhost)

login: Wed Aug 23 10:02:44 CDT 2017: iM file

FreeBSD/amd64 (localhost)

login:

The script specified by iM in /etc/gettytab for this system:

#!/bin/sh
echo "$(date): iM file"

[dab]

@vangyzen gave his approval out-of-band; he is having some Phabricator access issues right now and suggested I mention his approval here.

[wblock]

Man page changes look fine to me.