script add -T fmt to print time-stamps

script -r is useful for recording time-stamps of when output
happened. With -T, rather than playback the script in real-time
we simply print the time-stamps to show when the output happened.

This is very useful for example, for analyzing boot time activity.

If the fmt provided contains no % characters the default
%n@ %s [%Y-%m-%d %T]
is used, which lends itself to analysis by tools as well as humans.

Sponsored by: Juniper Networks, Inc.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D34511