lftpd.sh: move trap on top to catch early errors

Signed-off-by: Oleg Verych <olecom@gmail.com>
lib/cnl: open log for spawed lftpd.sh channel

stdout of lftpd.sh gets back into node.js (for inspection, wes)
and is being written into write stream of log file in `lftp.log`

* extend lftps[chancfg.id] consts: sh, log, wes
* common `lftp.wes` allows to get rid of double quit event

Signed-off-by: Oleg Verych <olecom@gmail.com>