prevent unicode-confusion in password by applying PRECIS, and username/email address by applying unicode NFC normalization

an é (e with accent) can also be written as e+\u0301. the first form is NFC,
the second NFD. when logging in, we transform usernames (email addresses) to
NFC. so both forms will be accepted. if a client is using NFD, they can log
in too.

for passwords, we apply the PRECIS "opaquestring", which (despite the name)
transforms the value too: unicode spaces are replaced with ascii spaces. the
string is also normalized to NFC. PRECIS may reject confusing passwords when
you set a password.

vendor/golang.org/x/text/internal/language/coverage.go

@@ -0,0 +1,28 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package language
+
+// BaseLanguages returns the list of all supported base languages. It generates
+// the list by traversing the internal structures.
+func BaseLanguages() []Language {
+	base := make([]Language, 0, NumLanguages)
+	for i := 0; i < langNoIndexOffset; i++ {
+		// We included "und" already for the value 0.
+		if i != nonCanonicalUnd {
+			base = append(base, Language(i))
+		}
+	}
+	i := langNoIndexOffset
+	for _, v := range langNoIndex {
+		for k := 0; k < 8; k++ {
+			if v&1 == 1 {
+				base = append(base, Language(i))
+			}
+			v >>= 1
+			i++
+		}
+	}
+	return base
+}