Split code into modules

This is a massive commit that restructures the code into modules:

db/
    All functions related to modifying the Database

types/
    All type definitions and methods that can be exclusivly used on
    these types without dependencies

policy/
    All Policy related code, now without dependencies on the Database.

policy/matcher/
    Dedicated code to match machines in a list of FilterRules

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/util/dns.go

@@ -0,0 +1,69 @@
+package util
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+const (
+	// value related to RFC 1123 and 952.
+	LabelHostnameLength = 63
+)
+
+var invalidCharsInUserRegex = regexp.MustCompile("[^a-z0-9-.]+")
+
+var ErrInvalidUserName = errors.New("invalid user name")
+
+// NormalizeToFQDNRules will replace forbidden chars in user
+// it can also return an error if the user doesn't respect RFC 952 and 1123.
+func NormalizeToFQDNRules(name string, stripEmailDomain bool) (string, error) {
+	name = strings.ToLower(name)
+	name = strings.ReplaceAll(name, "'", "")
+	atIdx := strings.Index(name, "@")
+	if stripEmailDomain && atIdx > 0 {
+		name = name[:atIdx]
+	} else {
+		name = strings.ReplaceAll(name, "@", ".")
+	}
+	name = invalidCharsInUserRegex.ReplaceAllString(name, "-")
+
+	for _, elt := range strings.Split(name, ".") {
+		if len(elt) > LabelHostnameLength {
+			return "", fmt.Errorf(
+				"label %v is more than 63 chars: %w",
+				elt,
+				ErrInvalidUserName,
+			)
+		}
+	}
+
+	return name, nil
+}
+
+func CheckForFQDNRules(name string) error {
+	if len(name) > LabelHostnameLength {
+		return fmt.Errorf(
+			"DNS segment must not be over 63 chars. %v doesn't comply with this rule: %w",
+			name,
+			ErrInvalidUserName,
+		)
+	}
+	if strings.ToLower(name) != name {
+		return fmt.Errorf(
+			"DNS segment should be lowercase. %v doesn't comply with this rule: %w",
+			name,
+			ErrInvalidUserName,
+		)
+	}
+	if invalidCharsInUserRegex.MatchString(name) {
+		return fmt.Errorf(
+			"DNS segment should only be composed of lowercase ASCII letters numbers, hyphen and dots. %v doesn't comply with theses rules: %w",
+			name,
+			ErrInvalidUserName,
+		)
+	}
+
+	return nil
+}