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/system.go

@@ -0,0 +1,61 @@
+package settings
+
+import (
+	"github.com/qdm12/gluetun/internal/configuration/settings/helpers"
+	"github.com/qdm12/gotree"
+)
+
+// System contains settings to configure system related elements.
+type System struct {
+	PUID     *uint16
+	PGID     *uint16
+	Timezone string
+}
+
+// Validate validates System settings.
+func (s System) validate() (err error) {
+	return nil
+}
+
+func (s *System) copy() (copied System) {
+	return System{
+		PUID:     helpers.CopyUint16Ptr(s.PUID),
+		PGID:     helpers.CopyUint16Ptr(s.PGID),
+		Timezone: s.Timezone,
+	}
+}
+
+func (s *System) mergeWith(other System) {
+	s.PUID = helpers.MergeWithUint16(s.PUID, other.PUID)
+	s.PGID = helpers.MergeWithUint16(s.PGID, other.PGID)
+	s.Timezone = helpers.MergeWithString(s.Timezone, other.Timezone)
+}
+
+func (s *System) overrideWith(other System) {
+	s.PUID = helpers.OverrideWithUint16(s.PUID, other.PUID)
+	s.PGID = helpers.OverrideWithUint16(s.PGID, other.PGID)
+	s.Timezone = helpers.OverrideWithString(s.Timezone, other.Timezone)
+}
+
+func (s *System) setDefaults() {
+	const defaultID = 1000
+	s.PUID = helpers.DefaultUint16(s.PUID, defaultID)
+	s.PGID = helpers.DefaultUint16(s.PGID, defaultID)
+}
+
+func (s System) String() string {
+	return s.toLinesNode().String()
+}
+
+func (s System) toLinesNode() (node *gotree.Node) {
+	node = gotree.New("OS Alpine settings:")
+
+	node.Appendf("Process UID: %d", *s.PUID)
+	node.Appendf("Process GID: %d", *s.PGID)
+
+	if s.Timezone != "" {
+		node.Appendf("Timezone: %s", s.Timezone)
+	}
+
+	return node
+}