move config-changing code from package mox-/ to admin/

needed for upcoming changes, where (now) package admin needs to import package
store. before, because package store imports mox- (for accessing the active
config), that would lead to a cyclic import. package mox- keeps its active
config, package admin has the higher-level config-changing functions.

store/lastknown.go

@@ -0,0 +1,52 @@
+package store
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/mjl-/mox/mox-"
+	"github.com/mjl-/mox/moxvar"
+	"github.com/mjl-/mox/updates"
+)
+
+// StoreLastKnown stores the the last known version. Future update checks compare
+// against it, or the currently running version, whichever is newer.
+func StoreLastKnown(v updates.Version) error {
+	return os.WriteFile(mox.DataDirPath("lastknownversion"), []byte(v.String()), 0660)
+}
+
+// LastKnown returns the last known version that has been mentioned in an update
+// email, or the current application.
+func LastKnown() (current, lastknown updates.Version, mtime time.Time, rerr error) {
+	curv, curerr := updates.ParseVersion(moxvar.VersionBare)
+
+	p := mox.DataDirPath("lastknownversion")
+	fi, _ := os.Stat(p)
+	if fi != nil {
+		mtime = fi.ModTime()
+	}
+
+	vbuf, err := os.ReadFile(p)
+	if err != nil && !os.IsNotExist(err) {
+		return curv, updates.Version{}, mtime, err
+	}
+
+	lastknown, lasterr := updates.ParseVersion(strings.TrimSpace(string(vbuf)))
+
+	if curerr == nil && lasterr == nil {
+		if curv.After(lastknown) {
+			return curv, curv, mtime, nil
+		}
+		return curv, lastknown, mtime, nil
+	} else if curerr == nil {
+		return curv, curv, mtime, nil
+	} else if lasterr == nil {
+		return curv, lastknown, mtime, nil
+	}
+	if strings.HasPrefix(moxvar.Version, "(devel)") {
+		return curv, updates.Version{}, mtime, fmt.Errorf("development version")
+	}
+	return curv, updates.Version{}, mtime, fmt.Errorf("parsing version: %w", err)
+}