Fix: Wrong poll events on UST application socket

The thread manage apps was adding UST sockets to the poll set with the
POLLIN event registered. However, the thread was not handling this event
which could ultimately cause an infinite loop if the application sends
data.

This has been observed with the bug386 when an application is stopped
and a lttng command is sent.

https://bugs.lttng.org/issues/386

Furthermore, a time window between the send and the reply recv of an UST
command was making the app manager loop actively because of a POLLIN
event on the socket caused by the reply from the application which is
finally handled a bit after by the client thread. This was not that
problematic but lead to a lot of repeated debug message and CPU time
lost.

This application thread is *only* handling error event usually triggered
by a close() on the UST socket thus OK to *not* wait for POLLIN/POLLOUT
event.

Signed-off-by: David Goulet <dgoulet@efficios.com>