* deploy: allow non-positional --force-mbr
Improvement on #182; fixed formatting, added comments, and made `[GPT partition index]` not fixed at `argv[2]` either. Incidentally, this allows `--force-mbr` and `[GPT partition index]` to coexist; there may be cause for a check for this.
* deploy: move usage to separate function
* deploy: fix style
* deploy: fix `usage` call
* limine-deploy: adjust usage information
* limine-deploy: add another explicit null check
