Add trace name handling throughout tracectl, ustcomm and ustcmd

This patch is the first real step in allowing multi-session
tracing. It pushes trace name all the way out to libustcmd
(though not yet to ustctl) and trace session support all
the way into tracectl in libust (though not yet down to markers).

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Acked-by: David Goulet <david.goulet@polymtl.ca>