chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/settings/errors.go

@@ -0,0 +1,51 @@
+package settings
+
+import "errors"
+
+var (
+	ErrCityNotValid                    = errors.New("the city specified is not valid")
+	ErrControlServerPrivilegedPort     = errors.New("cannot use privileged port without running as root")
+	ErrCountryNotValid                 = errors.New("the country specified is not valid")
+	ErrFirewallZeroPort                = errors.New("cannot have a zero port to block")
+	ErrHostnameNotValid                = errors.New("the hostname specified is not valid")
+	ErrISPNotValid                     = errors.New("the ISP specified is not valid")
+	ErrNameNotValid                    = errors.New("the server name specified is not valid")
+	ErrOpenVPNClientCertMissing        = errors.New("client certificate is missing")
+	ErrOpenVPNClientCertNotValid       = errors.New("client certificate is not valid")
+	ErrOpenVPNClientKeyMissing         = errors.New("client key is missing")
+	ErrOpenVPNClientKeyNotValid        = errors.New("client key is not valid")
+	ErrOpenVPNConfigFile               = errors.New("custom configuration file error")
+	ErrOpenVPNCustomPortNotAllowed     = errors.New("custom endpoint port is not allowed")
+	ErrOpenVPNEncryptionPresetNotValid = errors.New("PIA encryption preset is not valid")
+	ErrOpenVPNInterfaceNotValid        = errors.New("interface name is not valid")
+	ErrOpenVPNMSSFixIsTooHigh          = errors.New("mssfix option value is too high")
+	ErrOpenVPNPasswordIsEmpty          = errors.New("password is empty")
+	ErrOpenVPNTCPNotSupported          = errors.New("TCP protocol is not supported")
+	ErrOpenVPNUserIsEmpty              = errors.New("user is empty")
+	ErrOpenVPNVerbosityIsOutOfBounds   = errors.New("verbosity value is out of bounds")
+	ErrOpenVPNVersionIsNotValid        = errors.New("version is not valid")
+	ErrPortForwardingEnabled           = errors.New("port forwarding cannot be enabled")
+	ErrPortForwardingFilepathNotValid  = errors.New("port forwarding filepath given is not valid")
+	ErrPublicIPFilepathNotValid        = errors.New("public IP address file path is not valid")
+	ErrPublicIPPeriodTooShort          = errors.New("public IP address check period is too short")
+	ErrRegionNotValid                  = errors.New("the region specified is not valid")
+	ErrServerAddressNotValid           = errors.New("server listening address is not valid")
+	ErrSystemPGIDNotValid              = errors.New("process group id is not valid")
+	ErrSystemPUIDNotValid              = errors.New("process user id is not valid")
+	ErrSystemTimezoneNotValid          = errors.New("timezone is not valid")
+	ErrVPNProviderNameNotValid         = errors.New("VPN provider name is not valid")
+	ErrVPNTypeNotValid                 = errors.New("VPN type is not valid")
+	ErrWireguardEndpointIPNotSet       = errors.New("endpoint IP is not set")
+	ErrWireguardEndpointPortNotAllowed = errors.New("endpoint port is not allowed")
+	ErrWireguardEndpointPortNotSet     = errors.New("endpoint port is not set")
+	ErrWireguardInterfaceAddressNotSet = errors.New("interface address is not set")
+	ErrWireguardInterfaceNotValid      = errors.New("interface name is not valid")
+	ErrWireguardPreSharedKeyNotSet     = errors.New("pre-shared key is not set")
+	ErrWireguardPreSharedKeyNotValid   = errors.New("pre-shared key is not valid")
+	ErrWireguardPrivateKeyNotSet       = errors.New("private key is not set")
+	ErrWireguardPrivateKeyNotValid     = errors.New("private key is not valid")
+	ErrWireguardPublicKeyNotSet        = errors.New("public key is not set")
+	ErrWireguardPublicKeyNotValid      = errors.New("public key is not valid")
+
+	ErrUpdaterPeriodTooSmall = errors.New("VPN server data updater period is too small")
+)