Introduce emission site for syscall event rule

The tracer support multiple emission site configuration: entry, exit,
both.

On the CLI, the emission site is exposed directly on the event rule type
with the following syntax:

 --type=syscall:entry
 --type=syscall:exit
 --type=syscall:entry+exit

`--type=syscall` is equivalent to `--type=syscall:entry+exit`.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I65faea6872f121b98b781d931b6cc7301f20a0b6