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

cmd/syncthing/cli/main.go

@@ -61,23 +61,23 @@ func Run() error {
 	fakeFlags := []cli.Flag{
 		cli.StringFlag{
 			Name:  "gui-address",
-			Value: "URL",
-			Usage: "Override GUI address (e.g. \"http://192.0.2.42:8443\")",
+			Usage: "Override GUI address to `URL` (e.g. \"http://192.0.2.42:8443\")",
 		},
 		cli.StringFlag{
 			Name:  "gui-apikey",
-			Value: "API-KEY",
-			Usage: "Override GUI API key",
+			Usage: "Override GUI API key to `API-KEY`",
 		},
 		cli.StringFlag{
 			Name:  "home",
-			Value: "PATH",
-			Usage: "Set configuration and data directory",
+			Usage: "Set configuration and data directory to `PATH`",
 		},
 		cli.StringFlag{
-			Name:  "conf",
-			Value: "PATH",
-			Usage: "Set configuration directory (config and keys)",
+			Name:  "config",
+			Usage: "Set configuration directory (config and keys) to `PATH`",
+		},
+		cli.StringFlag{
+			Name:  "data",
+			Usage: "Set data directory (database and logs) to `PATH`",
 		},
 	}