I guess the original author of the popen() code didn't want to use our
<sys/queue.h> macro's, because the single linked list macro's didn't
offer O(1) deletion. Because I introduced SLIST_REMOVE_NEXT() some time
ago, we can now use the macro's here.
By converting the code to an SLIST, it is more consistent with other
parts of the C library and the operating system.
Reviewed by: csjp
Approved by: philip (mentor, implicit)