Verify that the configuration keys in the file are valid.

- Return an error if any of the keys don't match valid flags.
- Fix an issue ignoring merged values as named values.
- Fix tlsverify configuration key.
- Fix bug in mflag to avoid panics when one of the flag set doesn't have any flag.

Signed-off-by: David Calavera <david.calavera@gmail.com>

docs/reference/commandline/daemon.md

@@ -852,7 +852,7 @@ This is a full example of the allowed configuration options in the file:
 	"hosts": [],
 	"log-level": "",
 	"tls": true,
-	"tls-verify": true,
+	"tlsverify": true,
 	"tls-opts": {
 		"tlscacert": "",
 		"tlscert": "",