ihandle: Refactor ih_open to split out ih_attachfd

Refactor the function ih_open, so part of its logic gets split out
into the new ih_attachfd_r (and ih_attachfd) function. This allows
other code to splice in an existing fd, without going through the
normal "open" path.

This patch should incur no functional change; it is just code
reorganization.

Reviewed-on: http://gerrit.openafs.org/9968
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit f40e20d5671310500fb4f6acf38980e158a74c66)

Change-Id: I40bb515b5c822c3be3d3819c0183091753c41dea
Reviewed-on: http://gerrit.openafs.org/10176
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>