BitStructure: improve constructor argument processing

[pabigot]

This code:

const bs = lo.bits(lo.u8(), 'flags');
const str = lo.struct([bs]);

will result in failure to process the structure flags field because the intended property name is interpreted as the rarely-used second parameter boolean specifying the bit order, leaving the field without a property name and so with no way to represent the field content.

If the second parameter to lo.bits is a string and there is no third parameter, treat the second parameter as property and set msb to undefined.