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

This modification adds a new gettytab(5) option (iM) to specify a
program to run that will generate the initial (banner) message that is
displayed before the login prompt. Such a capability is useful when
dynamic information is needed in the banner message that cannot be
supplied by the set of % substitution sequences available in the "im"
option.

Reviewed by: vangyzen, wblock, manpages
Approved by: vangyzen (mentor)
MFC after: 1 week
Relnotes: yes
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D12108