Better factoring for flags code

[sunfishcode]

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.

[sunfishcode]

This will also involve eliminating some differences in types between the backends, where currently the linux_raw backend uses u32 for many of its flags types due to how bindgen works, and the libc backend uses i32 due to how C works.

[sunfishcode]

As of #701, the libc branch uses u32 for flags types. Some of the common code for flags is factored out, though there's more to do, though now, it won't require a semver bump.