Fix sockopt tests (#1534)

* Fix cfgs in sockopt tests

`rustix::time` and the txtime socket options are only available with the `time`
feature, but they are unconditionally used in sockopt tests. Mark them with
`cfg(feature = "time")` to enable compiling sockopt tests with just the `net`
feature enabled. Otherwise `cargo test --features net` would end up with a
compile error.

Fixes: 65b04ae ("Add support for SO_TXTIME / SCM_TXTIME (#1409)")

* Test `socket_passcred` only for Unix domain sockets

Since Linux 6.16, `SO_PASSCRED` is allowed only for `AF_UNIX`, `AF_NETLINK` and
`AF_BLUETOOTH` sockets. The tests used to get/set this option for IP sockets,
which doesn't work with new kernels, so make the tests use a Unix domain socket
instead.

On new kernels, the tests fail with:

```
---- sockopt::test_sockopts_ipv6 stdout ----

thread 'sockopt::test_sockopts_ipv6' panicked at tests/net/sockopt.rs:44:42:
called `Result::unwrap()` on an `Err` value: Os { code: 95, kind: Unsupported, message: "Operation not supported" }

---- sockopt::test_sockopts_ipv4 stdout ----

thread 'sockopt::test_sockopts_ipv4' panicked at tests/net/sockopt.rs:44:42:
called `Result::unwrap()` on an `Err` value: Os { code: 95, kind: Unsupported, message: "Operation not supported" }
```

The restriction was introduced to Linux in this commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7d8d93fdde50b86bbbf46a203c368ed320e729ab

Author: pekkarr

tests/net/sockopt.rs

@@ -9,10 +9,10 @@ use rustix::io;
     target_env = "newlib"
 ))]
 use rustix::net::ipproto;
-#[cfg(target_os = "linux")]
+#[cfg(all(target_os = "linux", feature = "time"))]
 use rustix::net::TxTimeFlags;
 use rustix::net::{sockopt, AddressFamily, SocketType};
-#[cfg(target_os = "linux")]
+#[cfg(all(target_os = "linux", feature = "time"))]
 use rustix::time::ClockId;
 use std::net::Ipv4Addr;
 use std::time::Duration;
@@ -40,8 +40,6 @@ fn test_sockopts_socket(s: &OwnedFd) {
     assert!(!sockopt::socket_broadcast(s).unwrap());
     // On a new socket we shouldn't have a linger yet.
     assert!(sockopt::socket_linger(s).unwrap().is_none());
-    #[cfg(linux_kernel)]
-    assert!(!sockopt::socket_passcred(s).unwrap());
 
     // On a new socket we shouldn't have an error yet.
     assert_eq!(sockopt::socket_error(s).unwrap(), Ok(()));
@@ -119,15 +117,6 @@ fn test_sockopts_socket(s: &OwnedFd) {
     // Check that we have a linger of at least the time we set.
     assert!(sockopt::socket_linger(s).unwrap().unwrap() >= Duration::new(1, 1));
 
-    #[cfg(linux_kernel)]
-    {
-        // Set the passcred flag;
-        sockopt::set_socket_passcred(s, true).unwrap();
-
-        // Check that the passcred flag is set.
-        assert!(sockopt::socket_passcred(s).unwrap());
-    }
-
     // Set the receive buffer size.
     let size = sockopt::socket_recv_buffer_size(s).unwrap();
     sockopt::set_socket_recv_buffer_size(s, size * 2).unwrap();
@@ -527,6 +516,18 @@ fn test_sockopts_ipv6() {
     test_sockopts_tcp(&s);
 }
 
+#[cfg(linux_kernel)]
+#[test]
+fn test_socket_passcred() {
+    crate::init();
+
+    let s = rustix::net::socket(AddressFamily::UNIX, SocketType::STREAM, None).unwrap();
+
+    assert_eq!(sockopt::socket_passcred(&s), Ok(false));
+    sockopt::set_socket_passcred(&s, true).unwrap();
+    assert_eq!(sockopt::socket_passcred(&s), Ok(true));
+}
+
 #[cfg(any(linux_kernel, target_os = "cygwin"))]
 #[test]
 fn test_socketopts_ip_mtu() {
@@ -668,7 +669,7 @@ fn test_sockopts_multicast_ifv6() {
 }
 
 #[test]
-#[cfg(target_os = "linux")]
+#[cfg(all(target_os = "linux", feature = "time"))]
 fn test_sockopts_txtime() {
     crate::init();