Better factoring for flags code

Rustix has this pattern where it defines flags types in both src/backend/linux_raw/\*/types.rs and src/backend/libc/\*/types.rs and then publicly exports those types from src/\*/\*.rs. This leads to a fair amount of duplication.

In many cases, espeically for flags corresponding to Linux-specific features, it would work to just define the public type in src/\*/\*.rs using the values of `c::*` constants which we could arrange to be defined by both backends, and it'd have less overall duplication.

It'd be nice to factor out the flags values in this way.