web: Validate if bucket names are reserved (#3841)

Both '.minio.sys' and 'minio' should be never allowed
to be created from web-ui and then fail to list it
by filtering them out.

Fixes #3840

cmd/web-handlers_test.go

@@ -265,6 +265,8 @@ func testMakeBucketWebHandler(obj ObjectLayer, instanceType string, t TestErrHan
 		{"", false},
 		{".", false},
 		{"ab", false},
+		{"minio", false},
+		{".minio.sys", false},
 		{bucketName, true},
 	}