Normalize CLI options to always use two dashes. (#8037)

Consistently use double dashes and fix typos -conf, -data-dir and
-verify.

Applies also to tests running the syncthing binary for consistency.

* Fix mismatched option name --conf in cli subcommand.

According to the source code comments, the cli option flags should
mirror those from the serve subcommand where applicable.  That one is
actually called --config though.

* cli: Fix help text option placeholders.

The urfave/cli package uses the Value field of StringFlag to provide a
default value, not to name the placeholder.  That is instead done with
backticks around some part of the Usage field.

* cli: Add missing --data flag in subcommand help text.

The urfave/cli based option parsing uses a fake flags collection to
generate help texts matching the used global options.  But the --data
option was omitted from it, although it is definitely required when
using --config as well.  Note that it cannot just be ignored, as some
debug stuff actually uses the DB:

syncthing cli --data=/bar --config=/foo debug index dump

lib/syncthing/utils.go

@@ -90,7 +90,7 @@ func LoadConfigAtStartup(path string, cert tls.Certificate, evLogger events.Logg
 			l.Infof("Now, THAT's what we call a config from the future! Don't worry. As long as you hit that wire with the connecting hook at precisely eighty-eight miles per hour the instant the lightning strikes the tower... everything will be fine.")
 		}
 		if originalVersion > config.CurrentVersion && !allowNewerConfig {
-			return nil, fmt.Errorf("config file version (%d) is newer than supported version (%d). If this is expected, use -allow-newer-config to override.", originalVersion, config.CurrentVersion)
+			return nil, fmt.Errorf("config file version (%d) is newer than supported version (%d). If this is expected, use --allow-newer-config to override.", originalVersion, config.CurrentVersion)
 		}
 		err = archiveAndSaveConfig(cfg, originalVersion)
 		if err != nil {