http: Optimize queued client aborts

Avoid sending unsent data and destroying otherwise legitimate sockets
for requests that are aborted while still in the agent queue. This
reduces stress on upstream systems who will likely respond to the
request but client app already knows that it will be dropped on the
floor and also helps avoid killing keep-alive connections.
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