slave-bin/*: cleanup and unify script style

Signed-off-by: Andreas Beckmann <anbe@debian.org>
slave-bin: factor out a file_age function

for getting the age of a file in seconds

Signed-off-by: Andreas Beckmann <anbe@debian.org>
detect_slave_problems: use idle-sleep from piuparts.conf

Signed-off-by: Andreas Beckmann <anbe@debian.org>
detect_slave_problems: move statefile to slave-directory

be independent from a master installation

Signed-off-by: Andreas Beckmann <anbe@debian.org>
detect_slave_problems: get rid of monitor-slave directory

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