Multiple fixes for enable/disable UST support

Makes the enable/disable command inside the main.c uses the event.c
functions. For this, the disable tracepoint functions (single and all)
for UST are added to event.c

Also fix the goto error path in event.c for disable/enable functions
where the UST event was removed from the hashtable if the event already
exist on the tracer side.

Fix the wrong hashtable being used for events lookup in ust_app.c for
enable/disable event for UST global domain.

Adds the disable event per PID function call in ust_app.c

At this commit, the "lttng disable-event -a -u" command segfault
lttng-ust at commit 5cd29ace704432c7a421f7749ef8591e30e1894c.

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