remove boilerplate code from maintainer scripts

Signed-off-by: Andreas Beckmann <anbe@debian.org>
*.postrm: silence rmdir

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s.deb: populate known_hosts with localhost's key

Signed-off-by: Andreas Beckmann <anbe@debian.org>
restrict sudoers permissions a little bit

Signed-off-by: Andreas Beckmann <anbe@debian.org>
master-command: drop explicit python

piuparts-master is executable and has a correct hashbang

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