sysctl: run sysctl service if /proc/sys/net is writable (#4425)

This simply changes this line:

    ConditionPathIsReadWrite=/proc/sys/

to this:

     ConditionPathIsReadWrite=/proc/sys/net/

The background for this is that the latter is namespaced through network
namespacing usually and hence frequently set as writable in containers, even
though the former is kept read-only. If /proc/sys is read-only but
/proc/sys/net is writable we should run the sysctl service, as useful settings
may be made in this case.

Fixes: #4370