sd-rtnl: minor fixes
path_check_timestamp: only keep the most recent timestamp

There is no point in keeping one timestamp for each directory, as we only
ever care about the most recent one.
udev: link-config - use _cleanup_ macro locally
rtnl: use _cleanup_ macro more
rtnl: use malloc0 instead of calloc
udev: link-config - use new0 instead of calloc
udev: link-config - use proper return values

Not sure if -ENOENT is the correct return value for when no persistent network
name is set, but couldn't think of anything better.
udev: link-config - use zero instead of memset
udev: link-config - use safe_atou instead of strtoul
rtnl: introduce default timeout

We set it to 10 secs (as we are only communicating with the kernel,
it seems we should be able to bail out sooner than sd-bus, which
uses 25).

When passing timout 0, the default is used, use this in link-config.
rtnl: complain if used after fork
udev: link-config - split connection to sockets from loading of configs

We want to load the config in _init, but not connect to the sockets before we are forked.
rtnl: cleanup socket_read_message