libustctl: use direct socket communication

This patch changes libustctl to be socket instead of pid oriented.
The user is expected to connect to a pid using
ustctl_connect_pid(pid_t) which returns a socket file-descriptor and
then use the socket for the rest of the api. This reduces the amount
of open and closing systemcalls and also makes it possible for a
session daemon to detect process shutdown by the socket closing.

David, this ones for you. :-)

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>