LibCore: Implement LocalSocket::release_fd

release_fd() releases the fd associated with the LocalSocket it is
called on. This is analogous to release_value() on container objects in
AK, after which the object does not contain the value.

Userland/Libraries/LibCore/Stream.cpp

@@ -535,4 +535,15 @@ ErrorOr<size_t> LocalSocket::read_without_waiting(Bytes buffer)
     return m_helper.read(buffer, MSG_DONTWAIT);
 }
 
+ErrorOr<int> LocalSocket::release_fd()
+{
+    if (!is_open()) {
+        return Error::from_errno(ENOTCONN);
+    }
+
+    auto fd = m_helper.fd();
+    m_helper.set_fd(-1);
+    return fd;
+}
+
 }