* Add support for precompressed zstd files (rfc8478)

* Avoid the hash lookup for the file extension.

* Only calculate Etag once

According to https://github.com/systemd/systemd/blob/v241/NEWS#L2799
The Directive ReadWriteDirectories= has been renamed to ReadWritePaths=
in 241.

Includes updating go.mod to use new module path

Currently, the instance waitgroup is decremented twice in `startServers()`: once when `Serve()` is finished and once when `ServePacket()` is finished. However, with a graceful shutdown, `Serve()` returns before the server has actually finished shutting down all active connections. This patch increases the wait group by one when the server is shut down so that the program only exits when all the server instances have finished serving their connections.

A random config is intended to be used only for solving TLS-ALPN
challenges; so we have to be sure to disable on-demand TLS so that
arbitrary names can't request certificates with another name's
on-demand config.

Slightly inconvenient because it uses int type and we use string, but
oh well. This fixes a bug related to setting -http-port and -https-port
flags which weren't being used by CertMagic in some cases.

This should help prevent hanging in some cases when the process is
restarted and tries to obtain or renew a certificate, for example, but
the lock remains from the previous shutdown (which was during the same
operation). Only works if the process is cleanly shut down with a signal
it can capture.

* verify client certs

* move client cert compatible checker to an independent function

* unexport client cert compatible checker

* rename functions and add comment

* gofmt code

* add test

* add back the comment

Each time the Caddyfile reloads and Caddy is restarted,
caddytls.NewConfig starts a goroutine for cleaning the
certificate storage. This goroutine ranges over a time.Ticker
channel; although Stop is called on this ticker, Stop does
not close the underlying channel so the goroutine never exits.

This change adds an additional channel that is listened to
in the certificate cleaning goroutine so it can exit
on restarts.

https://github.community/t5/How-to-use-Git-and-GitHub/Our-pull-request-templates-aren-t-showing-up-for-any-PRs/m-p/22958

* More informative rewrite test output

When running rewrite tests, the output in case of a test failure now
includes not only the rewritten URLs but also the from URL.

* Move re-escaping to regexpMatches

This commit moves the code to post-process the match replacements from
ComplexRule to regexpMatches, so it can later be re-used for SimpleRule.

Also changes the comment in an attempt to better explain the reasoning
behind that code.

The required strings.Replacer is now built only once.

* Support regex captures in simple rewrite rules

Closes #2586

* Lint: fix some errcheck #2541

* Lint: fix passing structcheck #2541

* Lint: update fix structcheck #2541

* Lint: fix errcheck for basicauth, browse, fastcgi_test #2541

* Lint: fix errcheck for browse, fastcgi_test, fcgiclient, fcgiclient_test #2541

* Lint: fix errcheck for responsefilter_test, fcgilient_test #2541

* Lint: fix errcheck for header_test #2541

* Lint: update errcheck for fcgiclient_test #2541

* Lint: fix errcheck for server, header_test, fastcgi_test, https_test, recorder_test #2541

* Lint: fix errcheck for tplcontext, vhosttrie_test, internal_test, handler_test #2541

* Lint: fix errcheck for log_test, markdown mholt#2541

* Lint: fix errcheck for policy, body_test, proxy_test #2541

* Lint: fix errcheck for on multiple packages #2541

- reverseproxy
- reverseproxy_test
- upstream
- upstream_test
- body_test

* Lint: fix errcheck in multiple packages mholt#2541
- handler_test
- redirect_test
- requestid_test
- rewrite_test
- fileserver_test

* Lint: fix errcheck in multiple packages mholt#2541

- websocket
- setup
- collection
- redirect_test
- templates_test

* Lint: fix errcheck in logger test #2541

run goimports against #2551
- lexer_test
- log_test
- markdown

* Update caddyhttp/httpserver/logger_test.go
Co-Authored-By: Inconnu08 <taufiqrx8@gmail.com>

* Update log_test.go

* Lint: fix scope in logger_test #2541

* remove redundant err check in logger_test #2541

* fix alias in logger_test #2541

* fix import for format #2541

* refactor variable names and error check #2541

* Update to match CertMagic's refactoring

* mod: CertMagic v0.5.0

Caddy can be built, even with plugins, without modifying the source
code and without special build scripts, thanks to Go modules. See
the README or wiki.

Fix instructions for building Caddy from source

* run go mod tidy

* import the right version of quic-go

Fix #2560

* Add link to Good First Issue

 in Contributing Section

* Fix heading

Temporary until Go 1.13