network: create runtime sub-directories after drop_privileges()

For old kernels not supporting AmbientCapabilities=, networkd is
started as root with limited capabilities. Then, networkd cannot
chown the directories under runtime directory as
CapabilityBoundingSet= does not contains enough capabilities.
This makes these directories are created after dropping privileges.
Thus, networkd does not need to chown them anymore.

Fixes #7863.