Fix: pid tracker should track "pgid" for noargs probes

The "pid" notion exposed by LTTng translates to the "pgid" notion in the
Linux kernel. Therefore using "current->pid" as argument to the PID
tracker actually ends up behaving as a "tid" tracker, which does not
match the intent nor the user-space tracer behavior.

The probes taking arguments were fixed by a prior commit, but it missed
probes without arguments.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>