Track the selected xtrn-section lightbar position

Fixes the scrolled/selected position when the sysop has more than screen-full of xtrn program sections. As reported by mlong.
14 jobs for master in 16 minutes and 25 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Build
passed jsdoor-freebsd #6844
FreeBSD

00:05:20

passed jsdoor-linux #6845
Linux

00:05:57

passed jsdoor-windows #6847
FreeBSD

00:03:41

passed sbbs-freebsd #6839
FreeBSD

00:06:29

passed sbbs-linux #6834
Linux

00:08:01

passed sbbs-windows #6837
Windows

00:06:52

passed sexpots-freebsd #6846
FreeBSD

00:00:10

passed sexpots-linux #6835
Linux

00:00:09

passed sexpots-windows #6838
Windows

00:00:48

passed syncdraw-freebsd #6842
FreeBSD

00:00:22

passed syncdraw-linux #6843
Linux

00:00:25

passed syncterm-freebsd #6840
FreeBSD

00:01:14

passed syncterm-linux #6836
Linux

00:01:52

passed syncterm-windows #6841
FreeBSD

00:02:24