bin/live-snapshot: Vastly rework script

This patch vastly reworks the live-snapshot script; it fixes a number of
bugs related to argument handling (such as ignored parameters), as well
as improving the general robustness of the operation.

The script can also be run successfully with set -e and set -u, pending
some other patches in 'scripts/live-helpers'

Signed-off-by: Chris Lamb <chris@chris-lamb.co.uk>