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/main.go

@@ -76,9 +76,9 @@ above). The value 0 is used to disable all of the above. The default is to
 show time only (2).
 
 Logging always happens to the command line (stdout) and optionally to the
-file at the path specified by -logfile=path. In addition to an path, the special
+file at the path specified by --logfile=path. In addition to an path, the special
 values "default" and "-" may be used. The former logs to DATADIR/syncthing.log
-(see -data-dir), which is the default on Windows, and the latter only to stdout,
+(see --data), which is the default on Windows, and the latter only to stdout,
 no file, which is the default anywhere else.