Replace `send` and `recv` flags with a more type-safe API

Currently send/recv functions just take them as an int and there are no defines.

At the very least there should be static definitions. But would probably be better as a proper 'Flag' type (enum?).

Start by locating the send and recv function signatures and the code that currently accepts integer flags. Review the surrounding Rust bindings and the issue discussion before choosing the type-safe API shape. Done means the flags have defined, type-safe representations and the affected calls remain usable.

Written by the indexing model from the issue text.