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/decrypt/decrypt.go

@@ -45,7 +45,7 @@ func (c *CLI) Run() error {
 	log.SetFlags(0)
 
 	if c.To == "" && !c.VerifyOnly {
-		return fmt.Errorf("must set --to or --verify")
+		return fmt.Errorf("must set --to or --verify-only")
 	}
 
 	if c.TokenPath == "" {