Fix enable syscall and bad value of poll set size

First, when enabling syscall tracing, on success 0 is return so we don't
add this value to the event list or else we were polling on fd 0 which
is not a valid event fd. A normal kernel create event, the fd of that
event is returned from the ioctl which is not the case for syscalls.

Second, the poll size was not reset when updating the kernel poll set so
at each kernel poll update, the value was not coherent with the real
poll size and thus creating unstable behavior (unknown).

This commit MIGHT fix a blocking problem happening when using a custom
channel and enabling syscall on it. This issue was reported by Julien
Desfossez <julien.desfossez@polymtl.ca>.

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