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.

mox-/txt.go

@@ -0,0 +1,24 @@
+package mox
+
+// TXTStrings returns a TXT record value as one or more quoted strings, each max
+// 100 characters. In case of multiple strings, a multi-line record is returned.
+func TXTStrings(s string) string {
+	if len(s) <= 100 {
+		return `"` + s + `"`
+	}
+
+	r := "(\n"
+	for len(s) > 0 {
+		n := len(s)
+		if n > 100 {
+			n = 100
+		}
+		if r != "" {
+			r += " "
+		}
+		r += "\t\t\"" + s[:n] + "\"\n"
+		s = s[n:]
+	}
+	r += "\t)"
+	return r
+}