MEDIUM: stream_interface: offer a generic function for connection updates

MEDIUM: stream_interface: offer a generic function for connection updates

We need to have a generic function to be called by upper layers when buffer
flags have been updated (the si->update function). At the moment, both sock_raw
and sock_ssl had their own which basically was a copy-paste. Since these
functions are only used to update stream interface flags, it is logical to
have them handled by the stream interface code.

This allowed us to remove the stream_interface-specific update function from
sock_raw and sock_ssl which now use the generic code.

The stream_sock_update_conn callback has also been more appropriately renamed
conn_notify_si() since it's meant to be called by lower layers to notify the
SI and possibly upper layers about incoming changes.