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

test/override_test.go

@@ -199,7 +199,7 @@ func TestOverrideIgnores(t *testing.T) {
 	log.Println("Starting sendOnly...")
 	sendOnly := syncthingProcess{ // id1
 		instance: "1",
-		argv:     []string{"-home", "h1"},
+		argv:     []string{"--home", "h1"},
 		port:     8081,
 		apiKey:   apiKey,
 	}
@@ -212,7 +212,7 @@ func TestOverrideIgnores(t *testing.T) {
 	log.Println("Starting sendRecv...")
 	sendRecv := syncthingProcess{ // id2
 		instance: "2",
-		argv:     []string{"-home", "h2"},
+		argv:     []string{"--home", "h2"},
 		port:     8082,
 		apiKey:   apiKey,
 	}