Fix ETA calculation.

Use remaining bytes divided by CPS rather than total estimated time
minus elapsed time.

We squirrley on resumed ZModem downloads.
14 jobs for dailybuild_linux-x64 in 7 minutes and 46 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Build
passed #24006
FreeBSD
jsdoor-freebsd

00:05:25

passed #24007
Linux
jsdoor-linux

00:05:31

passed #24009
FreeBSD
jsdoor-windows

00:02:41

passed #24001
FreeBSD
sbbs-freebsd

00:07:42

passed #23996
Linux
sbbs-linux

00:05:05

passed #23999
Windows
sbbs-windows

00:05:17

passed #24008
FreeBSD
sexpots-freebsd

00:00:20

passed #23997
Linux
sexpots-linux

00:00:16

passed #24000
Windows
sexpots-windows

00:00:24

passed #24004
FreeBSD
syncdraw-freebsd

00:00:37

passed #24005
Linux
syncdraw-linux

00:00:24

passed #24002
FreeBSD
syncterm-freebsd

00:01:32

passed #23998
Linux
syncterm-linux

00:02:10

passed #24003
FreeBSD
syncterm-windows

00:02:32