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/locations/locations.go

@@ -40,10 +40,10 @@ const (
 type BaseDirEnum string
 
 const (
-	// Overridden by -home flag
+	// Overridden by --home flag
 	ConfigBaseDir BaseDirEnum = "config"
 	DataBaseDir   BaseDirEnum = "data"
-	// User's home directory, *not* -home flag
+	// User's home directory, *not* --home flag
 	UserHomeBaseDir BaseDirEnum = "userHome"
 
 	LevelDBDir = "index-v0.14.0.db"
@@ -98,7 +98,7 @@ var locationTemplates = map[LocationEnum]string{
 	HTTPSCertFile: "${config}/https-cert.pem",
 	HTTPSKeyFile:  "${config}/https-key.pem",
 	Database:      "${data}/" + LevelDBDir,
-	LogFile:       "${data}/syncthing.log", // -logfile on Windows
+	LogFile:       "${data}/syncthing.log", // --logfile on Windows
 	CsrfTokens:    "${data}/csrftokens.txt",
 	PanicLog:      "${data}/panic-${timestamp}.log",
 	AuditLog:      "${data}/audit-${timestamp}.log",