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

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