LibCore: Implement LocalSocket::receive_fd and send_fd

These are just wrappers over the sendfd and recvfd syscalls.

Userland/Libraries/LibCore/Stream.cpp

@@ -6,6 +6,7 @@
  */
 
 #include "Stream.h"
+#include <LibCore/System.h>
 #include <fcntl.h>
 #include <netdb.h>
 #include <poll.h>
@@ -535,4 +536,24 @@ ErrorOr<NonnullOwnPtr<LocalSocket>> LocalSocket::connect(String const& path)
     return socket;
 }
 
+ErrorOr<int> LocalSocket::receive_fd(int flags)
+{
+#ifdef __serenity__
+    return Core::System::recvfd(m_helper.fd(), flags);
+#else
+    (void)flags;
+    return Error::from_string_literal("File descriptor passing not supported on this platform");
+#endif
+}
+
+ErrorOr<void> LocalSocket::send_fd(int fd)
+{
+#ifdef __serenity__
+    return Core::System::sendfd(m_helper.fd(), fd);
+#else
+    (void)fd;
+    return Error::from_string_literal("File descriptor passing not supported on this platform");
+#endif
+}
+
 }

Userland/Libraries/LibCore/Stream.h

@@ -410,6 +410,9 @@ public:
     virtual ErrorOr<void> set_blocking(bool enabled) override { return m_helper.set_blocking(enabled); }
     virtual ErrorOr<void> set_close_on_exec(bool enabled) override { return m_helper.set_close_on_exec(enabled); }
 
+    ErrorOr<int> receive_fd(int flags);
+    ErrorOr<void> send_fd(int fd);
+
     virtual ~LocalSocket() { close(); }
 
 private: