hwloc: ensure that an internal fd is marked as close-on-exec

Make sure that an internal, long-lived hwloc fd is marked as
close-on-exec so that children don't inherit it.  This patch is
committed upstream in the hwloc master and v1.9 branches as 7489287
and b654e19, respectively.  The patch applied here is the exact same
logic, but the surrounding code changed slightly since the hwloc v1.7
series, so the patch doesn't apply cleanly.

Refs #4550
oob_tcp_listener.c: set both ends of this thread to be close-on-exec

This pipe is used to communicate between threads in this process.
Mark both fd as close-on-exec so that children don't inherit this
pipe.

Refs #4550