Modularize unistd.h.
In swiftlang/swift-subprocess#293, it was discovered that some types
like pid_t and uid_t were leaking from a C shim library instead of from
the Glibc import. This meant that some public typealiases referencing
those types and the non-public library import were mismatching in
accessor visibility.
Figuring out what was awry wasn't immediately obvious from first
principles but on suggestion of swiftlang/swift#85229, adopting a
similar change for unistd.h for OpenBSD fixes the issue.
