introduced shared control request types
added control pipe method CONTROL_PIPE_REQ_LNS_ADD_MODIFY to modify LNS configuration
added control support for adding lns and status command in xl2tp-control
added status control command for lns, this returns tunnel and call information via the control socket
fixed potential null pointer when creating a new lns
refactored the do_control() method to use a handler approach for processing
added control method to remove an lns
modified lns_remove to close each call rather than just calling destroy_tunnel()
added the ability to add a pppd plugin and params to an lns
fixed issue with strtok modifying contents when pushing details for
pppd plugins.
added pppd plugin to pppd configuration when USE_KERNEL is true
I am a nutcase, did not realise that the pppd plugins could be loaded from the options file.

This commit reverts the work I did to allow plugins to be passed to pppd via xl2tpd
Merge pull request #53 from bluewhale/master

Introduced shared control request types, and modify method for LNS configuration