Fix missing events for all threads pollfd struct

The thread_quit_pipe event field of the pollfd was not set nor zeroed.
At high rate UST registration and modifying the pollfd set frequently,
the session daemon hang on the poll() call even if the thread_quit_pipe
was closed and explain by poll() "undefined behavior".

Also add a MSG() and ERR() statement after some errors.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>