MEDIUM: capture: Allow capture with slot identifier

MEDIUM: capture: Allow capture with slot identifier

This patch modifies the current http-request capture function
and adds a new keyword "id" that permits to identify a capture slot.
If the identified doesn't exists, the action fails silently.

Note that this patch removs an unused list initilisation, which seems
to be inherited from a copy/paste. It's harmless and does not need to
be backported.

   LIST_INIT((struct list *)&rule->arg.act.p[0]);