LibCore: Add CLocalServer, a server-only local socket class.

Instead of trying to support both client and server in CLocalSocket, let's
have a specialized server class.

The basic usage is:

    CLocalServer server;
    server.listen("/tmp/name-of-portal");
    server.on_ready_to_accept = [&] {
        CLocalSocket* client = server.accept();
        ...
    };

This will make things a lot simpler, since an accepting socket doesn't need
half of the stuff that a regular CIODevice provides. :^)

Libraries/LibCore/CLocalSocket.cpp

@@ -2,6 +2,14 @@
 #include <sys/socket.h>
 #include <errno.h>
 
+CLocalSocket::CLocalSocket(Badge<CLocalServer>, int fd, CObject* parent)
+    : CSocket(CSocket::Type::Local, parent)
+{
+    set_fd(fd);
+    set_mode(CIODevice::ReadWrite);
+    set_error(0);
+}
+
 CLocalSocket::CLocalSocket(CObject* parent)
     : CSocket(CSocket::Type::Local, parent)
 {