use local var for pid, seeing as we only just stored it in the shm
use childdata ptrs in main.c
dump_childnos: cache the index calculation instead of repeating it.

gcc is probably smart enough to figure this out, but it also makes
the code read easier
dump_childnos: convert to use childdata ptr

This removes the repeated index calc in a better way.
init_shm: use childdata & syscallrecord struct ptrs