Backport #25537

More fix for #24981

* #24981

Close #22361, #25552

* #22361
* #25552

There were many patches for Gitea's sub-commands to satisfy the facts:

* Some sub-commands shouldn't output any log, otherwise the git protocol
would be broken
* Sometimes the users want to see "verbose" or "quiet" outputs

That's a longstanding problem, and very fragile. This PR is only a quick
patch for the problem.

In the future, the sub-command system should be refactored to a clear
solution.

----

Other changes:

* Use `ReplaceAllWriters` to replace
`RemoveAllWriters().AddWriters(writer)`, then it's an atomic operation.
* Remove unnecessary `syncLevelInternal` calls, because
`AddWriters/addWritersInternal` already calls it.

# Why this PR comes

At first, I'd like to help users like #23636 (there are a lot)

The unclear "Internal Server Error" is quite anonying, scare users,
frustrate contributors, nobody knows what happens.

So, it's always good to provide meaningful messages to end users (of
course, do not leak sensitive information).

When I started working on the "response message to end users", I found
that the related code has a lot of technical debt. A lot of copy&paste
code, unclear fields and usages.

So I think it's good to make everything clear.

# Tech Backgrounds

Gitea has many sub-commands, some are used by admins, some are used by
SSH servers or Git Hooks. Many sub-commands use "internal API" to
communicate with Gitea web server.

Before, Gitea server always use `StatusCode + Json "err" field` to
return messages.

* The CLI sub-commands: they expect to show all error related messages
to site admin
* The Serv/Hook sub-commands (for git clients): they could only show
safe messages to end users, the error log could only be recorded by
"SSHLog" to Gitea web server.

In the old design, it assumes that:

* If the StatusCode is 500 (in some functions), then the "err" field is
error log, shouldn't be exposed to git client.
* If the StatusCode is 40x, then the "err" field could be exposed. And
some functions always read the "err" no matter what the StatusCode is.

The old code is not strict, and it's difficult to distinguish the
messages clearly and then output them correctly.

# This PR

To help to remove duplicate code and make everything clear, this PR
introduces `ResponseExtra` and `requestJSONResp`.

* `ResponseExtra` is a struct which contains "extra" information of a
internal API response, including StatusCode, UserMsg, Error
* `requestJSONResp` is a generic function which can be used for all
cases to help to simplify the calls.
* Remove all `map["err"]`, always use `private.Response{Err}` to
construct error messages.
* User messages and error messages are separated clearly, the `fail` and
`handleCliResponseExtra` will output correct messages.
* Replace all `Internal Server Error` messages with meaningful (still
safe) messages.

This PR saves more than 300 lines, while makes the git client messages
more clear.

Many gitea-serv/git-hook related essential functions are covered by
tests.

---------

Co-authored-by: delvh <dev.lh@web.de>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

* Second attempt at preventing zombies

* Ensure that the pipes are closed in ssh.go
* Ensure that a cancellable context is passed up in cmd/* http requests
* Make cmd.fail return properly so defers are obeyed
* Ensure that something is sent to stdout in case of blocks here

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint 2

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint 3

Signed-off-by: Andrew Thornton <art27@cantab.net>

* fixup

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Apply suggestions from code review

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

* Add debug option to serv to help debug problems

* fixup! Add debug option to serv to help debug problems

* AuthorizedKeysCommand should not query db directly

* Update routers/private/internal.go

* Fix import order