Follow the CLI refactoring
1. Remove the "checkCommandFlags" helper
2. Unify the web startup message, make them have consistent names as `./gitea help`
3. Fine tune some other messages (see the diff)
Replace #21790
And close #25965 by the way (it needs a separate fix for 1.20)
Major changes:
1. Move "gitea convert" to "gitea doctor conver". The old "gitea doctor"
still works as a hidden sub-command (to avoid breaking)
2. Do not write "doctor.log" by default, it's not useful in most cases
and causes bugs like 25965
3. Improve documents
4. Fix the "help" commands. Before, the "./gitea doctor" can't show the
sub-command help correctly (regression of the last cli/v2 refactoring)
After this PR:
```
./gitea help # show all sub-commands for the app
./gitea doctor # show the sub-commands for the "doctor"
./gitea doctor help # show the sub-commands for the "doctor", as above
```
Replace #10912
And there are many new tests to cover the CLI behavior
There were some concerns about the "option order in hook scripts"
(https://github.com/go-gitea/gitea/pull/10912#issuecomment-1137543314),
it's not a problem now. Because the hook script uses `/gitea hook
--config=/app.ini pre-receive` format. The "config" is a global option,
it can appear anywhere.
----
## ⚠️ BREAKING ⚠️
This PR does it best to avoid breaking anything. The major changes are:
* `gitea` itself won't accept web's options: `--install-port` / `--pid`
/ `--port` / `--quiet` / `--verbose` .... They are `web` sub-command's
options.
* Use `./gitea web --pid ....` instead
* `./gitea` can still run the `web` sub-command as shorthand, with
default options
* The sub-command's options must follow the sub-command
* Before: `./gitea --sub-opt subcmd` might equal to `./gitea subcmd
--sub-opt` (well, might not ...)
* After: only `./gitea subcmd --sub-opt` could be used
* The global options like `--config` are not affected
