Refactor random string stuff and seeding

Make sure we have a good random seed on the default RNG, that the
predictable RNG is clearly marked as such, that random strings are
actually the length requested, and that they contain a restricted set of
characters only.

cmd/syncthing/tls.go

@@ -19,11 +19,9 @@ import (
 	"bufio"
 	"crypto/rand"
 	"crypto/rsa"
-	"crypto/sha256"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
-	"encoding/binary"
 	"encoding/pem"
 	"io"
 	"math/big"
@@ -45,13 +43,6 @@ func loadCert(dir string, prefix string) (tls.Certificate, error) {
 	return tls.LoadX509KeyPair(cf, kf)
 }
 
-func certSeed(bs []byte) int64 {
-	hf := sha256.New()
-	hf.Write(bs)
-	id := hf.Sum(nil)
-	return int64(binary.BigEndian.Uint64(id))
-}
-
 func newCertificate(dir string, prefix string) {
 	l.Infoln("Generating RSA key and certificate...")