networkd: call networkd a "network management" rather "network configuration" tool

networkd: call networkd a "network management" rather "network configuration" tool

This has irked me for a while. For me network configuration is the stuff
we store on disk in configuration file. And networkd then *applies* the
configuration. But the units so far claimed that networkd was the
"configuration" itself. Which I guess might make sense to some, but to
me sounds a bit unprecise. Let's clean this up, and call what networkd
is doing "Network Management".