v.db.connect: Replace -o flag with the standard --overwrite

[HuidaeCho]

This PR fixes #3149 by using the standard --overwrite flag.

[marisn]

I'm sorry, but this shall not pass. You should not remove a flag in non-major release. Please add a printf with warning whenever -o is used: "The -o flag is deprecated and will be removed in future releases. Use --o instead" To future proof the code, wrap the old -o in a macro #if GRASS_VERSION_MAJOR < 9

[HuidaeCho]

This PR requires #3256.

[HuidaeCho]

@marisn, @landam Will this work with #3256?

[marisn]

@marisn, @landam Will this work with #3256?

Yes. I tested together with #3256 and backwards compatibility is in place.

[landam]

@HuidaeCho Please feel free to merge (you have more than zero approvals)...