Resolve "narrowing conversion from unsigned int ..." warnings

[Scottapotamas]

CLang Tidy warns many times per file about "narrowing conversion from unsigned int to int is implementation-defined".

This is mostly seen where an unsigned bitfield is used for error handling, and is assigned with enum values. Enum values default to signed int apparently.

There doesn't appear to be a legal cross-platform C90/C99 way to make enums unsigned, so we're left with defines or standard const integer defines. Could also rethink the use of a bitfield entirely, though dot-notation syntactically preferred to manually masking.