Update the display if the "command-line-specified" time format is

Update the display if the "command-line-specified" time format is
changed by updating those columns showing the time in the
"command-line-specified" format, not by redoing the entire packet list
display; that way, the display continues to show the same packets and
any packet the user selected remains selected.  (It's also less work to
do that - you don't have to re-dissect the packet.)

Turn "redisplay_packets()" into "filter_packets()", and do some other
cleanups.

svn path=/trunk/; revision=325