We need std.os.linux and std.c to agree on the types here, or else
we'd have to pointlessly cast across the difference up in the
std.posix wrapper.  I ran into this as a type error the first time
I tried to compile my code that calls posix.socketpair() on Linux
without libc.

All of our existing socket calls with these kinds of arguments in
std (including the existing c.socketpair as well as
os.linux.socket in this same file) use unsigned for all of these
parameters, and so this brings linux.socketpair() into alignment
with everything else.