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.

imapserver/fuzz_test.go

@@ -70,7 +70,7 @@ func FuzzServer(f *testing.F) {
 		f.Fatalf("open account: %v", err)
 	}
 	defer acc.Close()
-	err = acc.SetPassword(log, "testtest")
+	err = acc.SetPassword(log, password0)
 	if err != nil {
 		f.Fatalf("set password: %v", err)
 	}