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.

Change-Id: I89fd32377c123f2c55b77b01e1db4e14a199bf8e
Reviewed-on: http://gerrit.openafs.org/9968
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>