Main: Default --doctool path to '.' if none given

[akien-mga]

Many users miss the . part of the documentation that indicates godot --doctool . as the usage, so let's make it the default (as it used to be years ago).

[akien-mga]

Note that this doesn't try to be overly clever so if you run godot --doctool --no-docbase or godot --doctool -v it will dump the stuff in folders named --no-docbase or -v. To specify arguments after --doctool <path>, the path is still mandatory.

Edit: Or actually no, with -v it seems to work fine, and with --no-docbase it gives:

ERROR: Argument supplied to --doctool must be a valid directory path.
   at: start (main/main.cpp:1916)

So seems fine.

[akien-mga]

This was unused. No clue what it was supposed to do.

[YuriSizov]

Otherwise it works as expected on Windows for me and I see no regressions if I want it to output somewhere else.

[akien-mga]

For the reference, this is something that could be done further up after else if (i < (args.size() - 1)) { for all "paired" parameters, but it's more risky so I'm keeping it localized.

The proper fix anyway is to scrap all this and redo the argument parsing with a proper parser (#44594).

[YuriSizov]

Not sure what --no-docbase is supposed to do exactly, but it did something at least. So yeah, it works on Windows as expected.

[akien-mga]

Cherry-picked for 3.4.