Protect pidlist with a mutex to avoid a race causing a duplicate free()

Protect pidlist with a mutex to avoid a race causing a duplicate free()
when the same pipe FILE is pclosed()'d in different threads, and to avoid
corrupting the linked list when adding or removing items. The symptoms of
the linked list getting corrupted were pclose() either not finding the pipe
on the list, or the list becoming circular and pclose() looping infinitely.