slave_run: use piuparts.conf to get paths etc.

Signed-off-by: Andreas Beckmann <anbe@debian.org>
slave_run: ensure PIUPARTS_TMPDIR exists

Signed-off-by: Andreas Beckmann <anbe@debian.org>
slave_run: drop ancient trap+mail code

that's useless with daemonized screen

Signed-off-by: Andreas Beckmann <anbe@debian.org>