Turn off md5sum optionally if content-md5 is not set (#7609)

This PR also brings --compat option to run MinIO in strict
S3 compatibility mode, MinIO by default will now try to run
high performance mode.

cmd/common-main.go

@@ -161,6 +161,9 @@ func handleCommonCmdArgs(ctx *cli.Context) {
 	globalCertsCADir = &ConfigDir{path: filepath.Join(globalCertsDir.Get(), certsCADir)}
 
 	logger.FatalIf(mkdirAllIgnorePerm(globalCertsCADir.Get()), "Unable to create certs CA directory at %s", globalCertsCADir.Get())
+
+	// Check "compat" flag from command line argument.
+	globalCLIContext.StrictS3Compat = ctx.IsSet("compat") || ctx.GlobalIsSet("compat")
 }
 
 // Parses the given compression exclude list `extensions` or `content-types`.