std: fix linux uid_t, use uid_t/gid_t in std.os

- correct uid_t from i32 to u32 on linux
- define uid_t and gid_t for OSes missing definitions
- use uid_t/gid_t instead of plain u32s throughout std.os

lib/std/process.zig

@@ -578,8 +578,8 @@ fn testWindowsCmdLine(input_cmd_line: [*]const u8, expected_args: []const []cons
 }
 
 pub const UserInfo = struct {
-    uid: u32,
-    gid: u32,
+    uid: os.uid_t,
+    gid: os.gid_t,
 };
 
 /// POSIX function which gets a uid from username.
@@ -607,8 +607,8 @@ pub fn posixGetUserInfo(name: []const u8) !UserInfo {
     var buf: [std.mem.page_size]u8 = undefined;
     var name_index: usize = 0;
     var state = State.Start;
-    var uid: u32 = 0;
-    var gid: u32 = 0;
+    var uid: os.uid_t = 0;
+    var gid: os.gid_t = 0;
 
     while (true) {
         const amt_read = try reader.read(buf[0..]);