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	it fails, so don't re-close it here.  isc_createtcp will also
	set the fd attribute of the session; don't set it again.
(isc_createtcp): Relocate the fd.  Be careful to always close it
	if an error occurs.

	SO_LINGER and SO_REUSEADDR arguments.  The old-style style
	previously used should no longer be necessary on modern operating
	systems, and in fact it no longer works everywere.
(isc_listentcp): Clear errno on each loop in the retry loop.

any files.

Check HAVE_STRING_H and HAVE_STDLIB_H.

Don't try to set SO_DONTLINGER if SO_DONTLINGER is undefined.
Use O_NONBLOCK instead of FNDELAY if FNDELAY is undefined.