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
