slave_{run,join}: name the screen session "piuparts_slave_screen"

intentionally does not include the substring "piuparts-slave"

Signed-off-by: Andreas Beckmann <debian@abeckmann.de>
slave_{run,join,cleanup}: use pgrep to find processes

Signed-off-by: Andreas Beckmann <debian@abeckmann.de>
detect_leftover_processes: do not use master-directory

this is a *slave* script

delete the statefile daily and whine again

Signed-off-by: Andreas Beckmann <debian@abeckmann.de>