Feature: write "port" option to xl2tpd config. Check that this port is not used, if used - use "0" as port value.

replace NM_L2TP_KEY_USE_ACCOMP to NM_L2TP_KEY_NO_ACCOMP for consistency (may require user to re-save his ppp options)

Code much sychronized with network-manager-pptp (master 8b5c8b6ca3d24f7b5f736b8a7a29956f0bce35f3)
Now works with beeline-moscow tp.internet.beeline.ru