agent: Implement socket redirection.
* agent/gpg-agent.c (ENAMETOOLONG): New. (redir_socket_name, redir_socket_name_extra) (redir_socket_name_ssh): New. (remove_socket): Take care of the redir names. (main): Pass the redir names to create_server_socket. (create_socket_name): Remove length check - that is anyway done later. (create_server_socket): Add arg r_redir_name and implement redirection if Libassuan is at least 2.14.