Make "install page" respect environment config (#25648)

Replace #25580

Fix #19453

The problem was: when users set "GITEA__XXX__YYY" , the "install page"
doesn't respect it.

So, to make the result consistent and avoid surprising end users, now
the "install page" also writes the environment variables to the config
file.

And, to make things clear, there are enough messages on the UI to tell
users what will happen.

There are some necessary/related changes to `environment-to-ini.go`:

* The "--clear" flag is removed and it was incorrectly written there.
The "clear" operation should be done if INSTALL_LOCK=true
* The "--prefix" flag is removed because it's never used, never
documented and it only causes inconsistent behavior.


![image](https://github.com/go-gitea/gitea/assets/2114189/12778ee4-3fb5-4664-a73a-41ebbd77cd5b)

modules/setting/config_env.go

@@ -12,10 +12,31 @@ import (
 	"code.gitea.io/gitea/modules/log"
 )
 
+const (
+	EnvConfigKeyPrefixGitea = "GITEA__"
+	EnvConfigKeySuffixFile  = "__FILE"
+)
+
 const escapeRegexpString = "_0[xX](([0-9a-fA-F][0-9a-fA-F])+)_"
 
 var escapeRegex = regexp.MustCompile(escapeRegexpString)
 
+func CollectEnvConfigKeys() (keys []string) {
+	for _, env := range os.Environ() {
+		if strings.HasPrefix(env, EnvConfigKeyPrefixGitea) {
+			k, _, _ := strings.Cut(env, "=")
+			keys = append(keys, k)
+		}
+	}
+	return keys
+}
+
+func ClearEnvConfigKeys() {
+	for _, k := range CollectEnvConfigKeys() {
+		_ = os.Unsetenv(k)
+	}
+}
+
 // decodeEnvSectionKey will decode a portable string encoded Section__Key pair
 // Portable strings are considered to be of the form [A-Z0-9_]*
 // We will encode a disallowed value as the UTF8 byte string preceded by _0X and
@@ -87,7 +108,7 @@ func decodeEnvironmentKey(prefixGitea, suffixFile, envKey string) (ok bool, sect
 	return ok, section, key, useFileValue
 }
 
-func EnvironmentToConfig(cfg ConfigProvider, prefixGitea, suffixFile string, envs []string) (changed bool) {
+func EnvironmentToConfig(cfg ConfigProvider, envs []string) (changed bool) {
 	for _, kv := range envs {
 		idx := strings.IndexByte(kv, '=')
 		if idx < 0 {
@@ -97,7 +118,7 @@ func EnvironmentToConfig(cfg ConfigProvider, prefixGitea, suffixFile string, env
 		// parse the environment variable to config section name and key name
 		envKey := kv[:idx]
 		envValue := kv[idx+1:]
-		ok, sectionName, keyName, useFileValue := decodeEnvironmentKey(prefixGitea, suffixFile, envKey)
+		ok, sectionName, keyName, useFileValue := decodeEnvironmentKey(EnvConfigKeyPrefixGitea, EnvConfigKeySuffixFile, envKey)
 		if !ok {
 			continue
 		}