fix: promote unsigned integers to double to prevent overflow

[philtran]

Attempt to fix #412 , #171 , #132

Adds is_unsigned parameter to variable_type_from_field_type, and for unsigned ints (MYSQL_TYPE_LONG), will promote to MY_DBL instead of MY_INT32

[philtran]

@krlmlr Are you the best person to ask to look at/review this?

[krlmlr]

Thanks for the initiative and for your patience.

It's not clear to me that all users of the "unsigned" data type will want it to map to double, on the off chance that the values exceed 2^31-1. In other packages I've been exploring ways to configure handling of type conversion in an argument to dbConnect(). Let me think about how to standardize this; I'm also happy to review a proposal.