slave-bin: fix file_age() and return "0" for missing files

this was returning "0 0" ...

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

that really belongs to the slave, but as there is currently nothing
useful using this, drop it entirely

Signed-off-by: Andreas Beckmann <anbe@debian.org>
todo: #699160