ipaccess: initial works to get BTS mode working

This patch adds the initial support to get BTS mode working with
the ipaccess driver.

Now, the driver handles IPA ping, pong and id_ack messages
internally in BTS modes, and it passes the signalling messages
to the client application by invoking the callback line operations.

Moreover, with this patch, each IPA link object always has one
E1 line object associated.

Still HSL BTS-mode remains unimplemented.
4 files changed