lxc-ps: use posix shell and awk instead of bash

lxc-ps: use posix shell and awk instead of bash

Use awk to parse the output pf 'ps' and the tasks files for the
containers.

Use awk fields to find PID column rather than assume that the PID field
is exactly 5 chars wide and has a leading space ' PID'. This works as
long as the PID field is before the command or other field that include
spaces. This also makes it work with busybox 'ps'.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>