Atomic overlay file application in wwclient

Update wwclient such that each file in the runtime overlay is applied
atomically, and only if modified.

- Closes: #1307
- Closes: #1975
- Closes: #1226

Also adds --once flag to allow wwclient to be run one time.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

cmd/wwclient/main.go

@@ -13,7 +13,6 @@ func main() {
 
 	err := root.Execute()
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "ERROR: %s\n", err)
 		if wwclient.DebugFlag {
 			fmt.Printf("\nSTACK TRACE: %+v\n", err)
 		}