set $user to <unknown> if lsof could not tell the username

set $user to <unknown> if lsof could not tell the username

this happens e.g. on hosts that run containers in own user namespaces

without, the output will be full of warnings:
 Use of uninitialized value $user in hash element at dsa-check-libs line 188.

Signed-off-by: Peter Palfrader <peter@palfrader.org>