child_process: don't throw on EMFILE/ENFILE

EMFILE and ENFILE mean 'out of file descriptors'.  It's a run-time error
and as such should emit an error on the child process object, not throw
an exception.

Fixes #7453.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
test: fix up pummel/test-net-pingpong

Fix up a bad assumption in pummel/test-net-pingpong, namely that binding
to 'localhost' or '' means that incoming connections will have an IPv4
address.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
lint: lib/_http_client.js