Rename libustd to libustconsumer and ustd to ust-consumerd

This is a fargoing but necessary renaming of some ust components.
The point of the renaming is to allow for a new daemon, ustd, which
will be able to keep track of tracing sessions and connect applications
with consumers. Also, the current names were non-sensical.

This patch is a step on the way towards creating a session daemon
that can connect trace producers and consumers in a nice way.

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>