cmd/*, lib/tlsutil: Refactor TLS stuff (fixes #5256) (#5276)

This changes the TLS and certificate handling in a few ways:

- We always use TLS 1.2, both for sync connections (as previously) and
  the GUI/REST/discovery stuff. This is a tightening of the requirements
  on the GUI. AS far as I can tell from caniusethis.com every browser from
  2013 and forward supports TLS 1.2, so I think we should be fine.

- We always greate ECDSA certificates. Previously we'd create
  ECDSA-with-RSA certificates for sync connections and pure RSA
  certificates for the web stuff. The new default is more modern and the
  same everywhere. These certificates are OK in TLS 1.2.

- We use the Go CPU detection stuff to choose the cipher suites to use,
  indirectly. The TLS package uses CPU capabilities probing to select
  either AES-GCM (fast if we have AES-NI) or ChaCha20 (faster if we
  don't). These CPU detection things aren't exported though, so the tlsutil
  package now does a quick TLS handshake with itself as part of init().
  If the chosen cipher suite was AES-GCM we prioritize that, otherwise we
  prefer ChaCha20. Some might call this ugly. I think it's awesome.

lib/discover/global_test.go

@@ -110,9 +110,8 @@ func TestGlobalOverHTTPS(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// Generate a server certificate, using fewer bits than usual to hurry the
-	// process along a bit.
-	cert, err := tlsutil.NewCertificate(dir+"/cert.pem", dir+"/key.pem", "syncthing", 1024)
+	// Generate a server certificate.
+	cert, err := tlsutil.NewCertificate(dir+"/cert.pem", dir+"/key.pem", "syncthing")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -176,9 +175,8 @@ func TestGlobalAnnounce(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	// Generate a server certificate, using fewer bits than usual to hurry the
-	// process along a bit.
-	cert, err := tlsutil.NewCertificate(dir+"/cert.pem", dir+"/key.pem", "syncthing", 1024)
+	// Generate a server certificate.
+	cert, err := tlsutil.NewCertificate(dir+"/cert.pem", dir+"/key.pem", "syncthing")
 	if err != nil {
 		t.Fatal(err)
 	}