Start implementing conditionals for filters.

Start implementing conditionals for filters.

Allows to check the existence of a variable in predicates, making it
possible to trace syscall latency, as follow:

syscall:select:entry
{
  @start[pid] = nsecs;
}

syscall:select:return
/@start[pid]/
{
  @usecs = hist((nsecs - @start[pid]) / 1000);
  delete(@start[pid]);
}