Make --log-file-mode support the same mode format of chmod(1)

Make the command line parameter `--log-file-mode` support the same mode format of command `chmod(1)`. When pull request #193 added parameter `--log-file-mode`, it only supported octal numbers for simplicity. See the original discussion of this issue [here](https://github.com/AltraMayor/gatekeeper/pull/193#discussion_r273118531) for details.