std: complete C definitions for uid/gid setters
std: complete C definitions for uid/gid setters

Also:
- correct the definition of setgid
- update the existing definitions to use the uid_t/gid_t types

andrewrk/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig