lib/upgrade: Auto upgrade signature should cover version & arch (fixes #3044)

New signature is the HMAC of archive name (which includes the release
version and architecture) plus the contents of the binary. This is
expected in a new file "release.sig" which may be present in a
subdirectory. The new release tools put this in [.]metadata/release.sig.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3043

lib/upgrade/upgrade_unsupp.go

@@ -14,7 +14,7 @@ func upgradeTo(binary string, rel Release) error {
 	return ErrUpgradeUnsupported
 }
 
-func upgradeToURL(binary, url string) error {
+func upgradeToURL(archiveName, binary, url string) error {
 	return ErrUpgradeUnsupported
 }