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/cli_test.go

@@ -31,7 +31,7 @@ func TestCLIReset(t *testing.T) {
 
 	// Run reset to clean up
 
-	cmd := exec.Command("../bin/syncthing", "-no-browser", "-home", "h1", "-reset-database")
+	cmd := exec.Command("../bin/syncthing", "--no-browser", "--home", "h1", "--reset-database")
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stdout
 	err := cmd.Run()
@@ -63,9 +63,9 @@ func TestCLIGenerate(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// -generate should create a bunch of stuff
+	// --generate should create a bunch of stuff
 
-	cmd := exec.Command("../bin/syncthing", "-no-browser", "-generate", "home.out")
+	cmd := exec.Command("../bin/syncthing", "--no-browser", "--generate", "home.out")
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stdout
 	err = cmd.Run()
@@ -91,7 +91,7 @@ func TestCLIFirstStartup(t *testing.T) {
 
 	// First startup should create config, BEP certificate, and HTTP certificate.
 
-	cmd := exec.Command("../bin/syncthing", "-no-browser", "-home", "home.out")
+	cmd := exec.Command("../bin/syncthing", "--no-browser", "--home", "home.out")
 	cmd.Env = append(os.Environ(), "STNORESTART=1")
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stdout