dco: only pass struct context to init function

dco: only pass struct context to init function

Future DCO code will require accessing the `multi` member of the
context object.

For this reason a pointer to the context has to be stored in the
DCO context along with the rest.

At this point, rather than making the call to ovpn_dco_init()
longer with more and more parameters, pass the struct context
only and let the implementation extract the needed fields.

Change-Id: I673a17f8c5dec66cc6c28c1ed44780a7a63927d7
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>