ihandle: Make _ONCLOSE the sync behavior default

The _DELAYED behavior has had serious problems in the past, so change
the default to be _ONCLOSE instead.

This is a 1.6-only change. On master, the _DELAYED option does not
exist at all, and the _ONCLOSE behavior was made the default when this
option was introduced in master, in commit
eb5190eb4a7cd95166866a89e0a8f3a69bbc6e8f.

Change-Id: I01a50e1d829c141c38fbbbaba2c6d2d5a371b130
Reviewed-on: http://gerrit.openafs.org/10809
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>